Last night I made a Rose Elliott recipe from my battered old copy of her Supreme Vegetarian Cookbook. (We're not vegetarians but we don't eat much meat.) It doesn't sound promising and it's far from posh nosh, but it's very tasty and ticks several other boxes for me: not expensive, healthy, easy, doesn't need lengthy or difficult preparation, cooks unattended, only needs one large casserole dish that you can use on the hob, so minimal washing up, reheats well. Could be frozen too, I think.
For 4-6 portions, you need a little oil, 100g whole or red lentils (uncooked weight), 3 or 4 brown-skinned onions, 5 or 6 good sized carrots, 4 largeish potatoes or equivalent in smaller ones, a few celery sticks, 2-300g mushrooms, vegetable stock (2 tbs vegetable bouillon powder or use cubes, 1 litre of boiling water), 4 tbs soy sauce, 1 tbs Lea & Perrin's or veggie equivalent, a couple of bayleaves, a little dried thyme and sage or other dried herbs, plain flour. I served it with steamed kale. I do think it needs a green vegetable on the side.
Wash the potatoes (no need to peel unless you want to). Top, tail and peel the onions and carrots, chop roughly. Wash and trim the celery stalks, roughly slice. Heat the oil in the casserole dish on a fairly high heat. Add the onions, fry for a few minutes. Add the carrots and celery, stir. Chop the potatoes into chunks (about the maximum size you'd want to spear with a fork, if you see what I mean) and add to the casserole with the lentils, flour, bayleaves, herbs, soy sauce, Lea & Perrin's, stock. Stir well. Bring to the boil, turn the heat right down so it simmers but doesn't burn on the bottom, partially cover the dish, leave to cook for 20 minutes, stirring occasionally, then add sliced mushrooms, bring back to simmer, leave to cook for another 20 minutes (could be a a bit less if using red lentils as they cook more quickly). Check all vegetables and lentils are cooked. Serve hot with a green vegetable, or as it comes with chopped parsley on the top for colour.