Cook the Bacon
In a large skillet, cook bacon until crispy. Drain on paper towels and chop into small pieces.

Blanch the Green Beans
Bring a large pot of salted water to a boil. Cook green beans for 4 to 5 minutes until tender-crisp. Drain and transfer to an ice bath to stop the cooking. Set aside.

Make the Cream Sauce
In the same skillet, melt butter over medium heat. Add onion and mushrooms, sautéing for 5 to 7 minutes. Add garlic and cook 1 more minute. Sprinkle in the flour and stir until absorbed. Gradually whisk in milk and broth. Simmer until thickened, about 4 to 5 minutes. Season with salt, pepper, and optional paprika.

Assemble the Casserole
In a large mixing bowl, combine green beans, sauce, chopped bacon, and half of the shredded cheese. Transfer to a greased 9×13-inch baking dish.

Add Topping and Bake
Sprinkle with remaining cheese and all of the crispy onions. Bake uncovered for 25 to 30 minutes until hot and bubbly.

Serve Warm
Let rest for 5 minutes before serving. Best served fresh from the oven.