Cut the potatoes into thirds and they will cook faster. Cover with water, bring to a boil, and simmer until a fork pokes easily through them. The smaller the pieces, the faster they'll cook. If you're not in a hurry, you can leave them whole. Drain when done.

Whip the potatoes with the milk. Add the sour cream, salt, pepper, onions if you're using them, and 3/4 cup of cheese. Beat together. I use my big standing mixer for this, but if you've got one, use a hand mixer in the same pot as the potatoes cook in and save a dish.

Butter a 9x9 pan or small casserole dish. Spread the mashed potatoes in it, cover with the remaining grated cheese, and sprinkle paprika over the top. Cover and refrigerate if you want. Bake at 350 for 20-30 minutes (depending on whether the potatoes got cold or not) and serve.