Simultaneously prepare the lentils, butternut squash, potatoes, and mushrooms:
Pick through the lentils to remove debris, then rinse them thoroughly.
Cook the lentils in a saucepan with water until just done, about 35 minutes.
Peel and cube the butternut squash.
Put the squash cubes in a saucepan, cover with water, bring to a boil, then simmer until just barely soft, about 15 minutes.
At the same time, rinse, peel, and chop the potatoes.
Put the potatoes in the larger pot, cover with water, and cook until done but not falling apart. The amount of time needed depends on the size of your potatoes chunks.
Simultaneously, rinse the quinoa thoroughly, then put it in a saucepan with twice as much water as quinoa, bring to a boil, lower heat to simmer, cover and cook for 15 minutes, remove from burner, and let sit (covered) for 10 minutes.
Put cooked lentils, squash, and quinoa in a bowl, and mix thoroughly.
Small dice the onion, and slice the mushrooms.
Heat the iron or other skillet over medium-high heat, adding enough oil to cover the bottom (about 1 tbsp for a 10-inch pan).
Sauté the onion in the oil until it softens, approximately 4 minutes.
Add the sliced mushrooms to the onions and continue to sauté until the mushrooms start to soften, about 3 minutes.
Mince the garlic, then add to the onion-mushroom mixture and continue to sauté for about 30 seconds-1 minute, or until the garlic is fragrant.
Add the tomato paste, lentil mixture, and enough vegetable broth to keep mixture from sticking.
Sprinkle the nutritional yeast, as well as 1 tsp of both salt and pepper, on the mixture, then mix well. If the mixture is very soupy, leave on simmer to reduce the sauce.
Drain the potatoes, then mash well. Warm the milk and butter in the microwave or on the stove, then add to the mashed potatoes along with ¾ tsp each of salt and pepper. Mix thoroughly.
Put the lentil mixture in the bottom of the casserole dish, then layer the mashed potatoes on top.
Finish in a 400F/200C oven for about 20 minutes, or until the top is slightly browned.
Serve with a simple green salad and garden peas.