Roast Vegetable and Lentil Salad

What this is

Roast beetroot, carrot and red onion tossed through cooked green lentils with a lemon and mustard dressing, feta and herbs. Fifty minutes, six servings, most of it in the oven.

It is good warm and better at room temperature, which makes it the best packed-lunch dish here.

The research below is mostly about iron, because lentils and beetroot are both foods people eat for it and both come with a complication.

Ingredients

For 6 servings

3 beetroot, peeled and cut into wedges 3 carrots, cut into batons 2 red onions, cut into wedges 3 tbsp olive oil 1 tsp cumin seeds Salt and black pepper

250 g dry green or Puy lentils, or 2 tins, drained 1 bay leaf

Dressing 4 tbsp olive oil Juice of 1 lemon 1 tbsp Dijon mustard 1 clove garlic, minced 1 tsp honey Salt and pepper

To finish 100 g feta, crumbled Large handful of parsley and dill 50 g toasted walnuts or pumpkin seeds

Method

1. Heat the oven to 200C. Toss the beetroot, carrot and onion with the oil, cumin seeds and seasoning, spread on a tray in one layer, and roast 40 minutes until the edges caramelise.

2. Meanwhile simmer the lentils with the bay leaf in unsalted water for 20 to 25 minutes until tender but holding shape. Salt them only at the end; salting early can toughen the skins. Drain well.

3. Whisk the dressing.

4. Dress the lentils while they are still warm. Warm pulses absorb dressing; cold ones sit in it.

5. Fold through the roast vegetables.

6. Finish with feta, herbs and toasted nuts. Do not stir the beetroot in too early or the whole thing turns pink, unless you want that.

The iron in lentils is complicated, and processing changes it

This is the most interesting study in the piece, because it measured absorption rather than content.

Researchers assessed the impact of boiling and fermentation on total phenolic content, total dietary fibre, and the bioaccessibility and bioavailability of iron, zinc, copper, calcium, sulphur and magnesium in iron-biofortified lentils, using an in vitro model. Iron bioavailability was measured by ferritin formation in a Caco-2 cell bioassay.

A Caco-2 cell assay is a layer of human intestinal cells in a dish. It is not a person, and it is a considerable step closer than simply measuring what is in the food.

Fermented lentil flour had the highest phenolic content, 331.3 mg per 100 g, and the lowest dietary fibre at 20.1 percent. Iron bioaccessibility was highest in the fermented flour at 76.1 percent, while copper at 99.0 percent, zinc at 62.6 percent, calcium at 60.2 percent, magnesium at 22.9 percent and sulphur at 22.6 percent were all higher in the boiled flour.

Read that again: fermentation won for iron, boiling won for everything else. There is no single best processing method, which is not the tidy answer anybody wants.

Bioavailability followed the same split, with iron at 69.4 percent and calcium at 50.3 percent highest in fermented flour, and zinc, copper, sulphur and magnesium higher in boiled.

Boiling is what this recipe does, and by these numbers that favours the zinc, copper and calcium.

This was an in vitro digestion model with a cell bioassay, on iron-biofortified lentil flour, not whole boiled lentils in a salad. In vitro results do not always hold in people, and the lentils tested were bred for higher iron.

What pressure cooking does to beetroot

Beetroot has a similar story and the study on it is unusually practical.

Researchers tested pressure cooking, steaming and open pan processing across three beetroot variants, juice, residue, and juice with residue, aiming to enhance iron retention and the factors that help iron absorption.

Folate at 31.33 to 38.33 micrograms, beta-carotene at 12.33 to 21.00 micrograms and saponin at 12.66 to 17.33 mg per 100 g dry weight were best retained in the juice-with-residue variant. Iron was retained most in the residue variant, followed by juice with residue.

Residue means the fibrous solids. So the part people throw away when juicing is where the iron was.

The finding that changes how you cook it: iron absorption inhibitors including oxalates, tannins and calcium were reduced significantly after pressure cooking.

And the honest cost, which the authors report rather than bury: reductions were also observed in bioactive compounds including nitrates, betanin and vulgaxanthin.

So pressure cooking removed the things blocking iron absorption and removed some of the things people eat beetroot for. That is a trade, not a win, and roasting sits somewhere between the extremes.

This measured composition and retention in processed beetroot, not absorption in people, and it compared pressure cooking, steaming and open pan rather than roasting.

Why the lemon is in the dressing

A review of vegetable cooking methods gives the general principle behind everything above.

The review compared the sous-vide method against conventional heat treatments including boiling in water, steaming, pressure cooking, microwave cooking, baking, grilling and cook-vide.

Its finding: sous-vide usually allowed retention of greater amounts of vitamins, especially vitamin C, along with phenolic compounds and minerals, giving higher nutritional value and bioavailability of bioactive ingredients. The authors attribute this to maintaining a controlled low temperature in a vacuum, reducing loss of water and volatile components.

Controlled low temperature and less water contact. Those are the two variables, and roasting has one of them: no water for things to leach into, at the cost of higher heat.

Vitamin C is the thread back to the dressing. It is the nutrient most easily destroyed by heat, and it also improves absorption of non-heme iron, which is the form found in lentils and beetroot.

That is why the lemon goes in raw at the end rather than into the roasting tray. Adding a raw acidic ingredient to an iron-rich plant dish is a documented pairing, and heating it away defeats the point.

The review concerns sous-vide against other methods and did not test this dish. The vitamin C and iron absorption pairing is established nutrition rather than a finding of any study cited here, and I am stating it as background rather than as evidence from these sources.

Storage and swaps

Keeps 4 days in the fridge, dressed, and genuinely improves on day two. It is the rare salad that does.

Do not freeze it. Roast vegetables collapse and lentils go mealy.

Add the herbs and nuts per portion rather than all at once, or they go limp and soft respectively.

Swap green lentils for Puy, beluga or brown. Do not use red lentils; they disintegrate.

Swap the roast vegetables for squash, parsnip, cauliflower or fennel, adjusting time.

Swap feta for goat cheese, or leave it out and add extra nuts.

And dress the lentils while warm. It is the single biggest difference between this being good and being a bowl of cold pulses.