Preheat oven to 350°F; grease a 9x13-inch dish.
Boil beans in a large pot of salted water 5 minutes until crisp-tender; drain and set aside.
Melt butter in a large skillet over medium-high; cook mushrooms 5-7 minutes until golden and liquid evaporates.
Cook garlic 1 minute; sprinkle flour over mushrooms, stir to coat, and cook 1 minute.
Gradually whisk in broth and cream; simmer 3-5 minutes until thickened and stir in soy sauce, salt, and pepper.
Stir beans and 1 cup fried onions into sauce; transfer to dish.
Bake 20 minutes; top with remaining onions and bake another 5-10 minutes until onions are golden and casserole bubbly.