Get ahead: Assemble the casserole without the onion topping, cover, and refrigerate up to 1 day in advance. Let the casserole sit at room temperature while the oven heats up. Remove the lid and top with fried onions. Bake for 30-40 minutes, but if the onions are browning too quickly, cover the top with aluminum foil during the last 10 minutes.
Replacement: You can use 3 cans (about 15 ounces) of green beans in place of fresh green beans. Drain well before adding to mushroom sauce.
storage: Leftovers can be refrigerated in an airtight container for up to 4 days.