This is adapted from a recipe in Mollie Katzen's Vegetable Heaven. It's not a lot of work, but it does require attention every now and then for a couple of hours.
I've become somewhat particular about lentils, I must confess. I really don't like the supermarket green/brown lentils very much anymore, particularly in soup. They fall apart and the skins kind of bug me. I'd much rather eat those snooty "French" lentils, sometimes called lentils de puy, for soup or pretty much anything else. This is one recipe that works really well with the plain, ordinary (cheap!) brown lentils, though. I've never tried this with red lentils, though I bet it would be pretty good.
1 1/2 cups lentils, cooked in 4 cups water
3-4 good sized onions, chopped fairly fine
2 T olive oil (or a combination of olive oil and butter)
1 T (or maybe a little more) balsamic vinegar
2-4 T brown sugar
1-2 cups diced tomatoes (amount variable, also kind of tomatoes variable -- I've successfully used crushed tomatoes, sauce, leftover pizza sauce . . . )
salt and pepper to taste
Heat olive oil over medium heat and add onions and several pinches of salt. Stir every few minutes for 10-15 minutes, until they start to get brown around the edges. Then put a cover on the pot and turn the heat down to low. Stir only every 15 minutes or so until the onions are nice and brown. (When I take the lid off, I drain the condensed steam off the lid but not back into the pot. The onions will brown better with the liquid removed.)
Add the tomatoes and bring to a simmer. Keep simmering, uncovered, until the tomatoes reduce and meld with the onions, making sort of a syrupy sauce. Then add the remaining ingredients, starting with the smaller amounts of vinegar and sugar. Don't be afraid to add more, though. Stir well. Taste for salt; it might need quite a bit.
This is not a particularly pretty dish, but it is delicious.
I served it with a rice and broccoli raab gratin: not terribly gorgeous, but warming and good comfort food.