Green Bean Casserole

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2 tablespoons unsalted butter, plus more for the baking dish

1/2 cup chopped onions

2 cups sliced mushrooms

One 10.5-ounce can cream of mushroom soup

1 cup heavy cream

1 cup shredded white Cheddar

Kosher salt and freshly ground black pepper

One 16-ounce bag frozen cut or French-style green beans, thawed

One 2.8-ounce can fried onion rings