Kosher salt
green beans, trimmed
(1 1/2 stick) unsalted butter, divided
yellow onions, thinly sliced
cremini mushrooms, sliced
Freshly ground black pepper
cloves garlic, finely chopped
all-purpose flour
whole milk
French's fried onions
2 (14.5 ounce) cans green beans, drained
1 (10.5 ounce) can condensed cream of mushroom soup
1 cup shredded Cheddar cheese, divided
1 (6 ounce) can French-fried onions
Preheat the oven to 350 degrees F (175 degrees C).
Mix green beans and condensed soup together in a large microwave-safe bowl until well combined. Microwave on high until warm, 3 to 5 minutes.
Stir 1/2 cup Cheddar cheese into the green bean mixture. Microwave on high for 2 to 3 minutes.
Transfer mixture to a casserole dish and spread evenly over the bottom. Sprinkle French-fried onions over top, then sprinkle with remaining Cheddar.
Bake in the preheated oven until cheese is melted and the onions are just turning brown, about 10 minutes.