For the lentils: Boil the lentils in water and salt until tender, about 20 minutes. Drain. Sauté chopped onions in the oil for about 3 minutes, and add the chopped red pepper for about 5 more minutes, add salt and pepper. Add the lentils, lower the heat (lentils are already cooked).
Season with a lot of cumin, a little bit of cayenne pepper, salt and pepper. You can try them and keep seasoning until you like them.
For the mashed potatoes: Peel (before or after boiling) and boil the potatoes in water with salt. Once tender, mash with fork. While you mash you can add the butter, the milk, salt, parmesan cheese, pepper and nutmeg. It doesn't have to be neither smooth or hard, something in between, you can control this with the milk. Don't add the egg until the end so you can try it and season as you like. Once mashed and once you love it and can't stop eating it, add the egg and mix well.
Use a baking dish or casserole dish (I used a glass Pyrex), spray it with nonstick spray, and add one thin layer of the mashed potatoes, less than half an inch tall. Spread it evenly. Add the lentils on top of it, spread them, and add the rest of the mashed potatoes on top of the lentils. Draw lines with a fork on top of the mashed potatoes, these will brown and be crispy.
Bake pastel for 20-30 minutes, until the lines of the fork turn a little golden.