Make-Ahead Mashed Potatoes with Buttery Onions

How to Make Make-Ahead Mashed Potatoes with Buttery Onions

a.k.a. Casserole Baked Mashed Potatoes (perfect for Thanksgiving)

#becauseittastesgood

Mashed Potatoes:
1.5 Lb yukon gold potatoes
4 Tbsp butter
1/3 cup sour cream
1/3 cup whole milk
Salt and pepper

Onion Topping:
2-3 Tbsp butter
1.5 yellow onion, diced (https://youtu.be/Nmr1l5IV9Os)