I used 2 tbsp butter, browned it and added a bit of avocado oil to start.
Cooked down the onions a bit, FIRST! Then added mushrooms.
Making day of - stir in a portion of fried onions, follow oven instructions. Add top onions and bake another 5-10 minutes for the sexy, crispy top.
Prepped for hosting elsewhere -
Complete steps up to adding fried onions.
At Location - When it's warmed, remove the dish, add and stir fried onions into casserole and toppong onions. Put back in oven for 5-10 minutes.
