Preheat oven to 375°F. Lightly grease a 9x13-inch baking dish.
Cook bacon in a large skillet over medium heat until crisp, 7–9 minutes. Transfer to paper towels. Reserve 2 tablespoons drippings.
Blanch green beans in salted boiling water until bright and crisp-tender, 4–5 minutes. Drain, rinse cold, and pat dry.
Sauté onion and mushrooms in the bacon drippings with 2 tablespoons butter over medium heat until softened and lightly browned, 6–8 minutes.
Stir in garlic and cook until fragrant, about 30 seconds.
Sprinkle in flour and cook, stirring, 1 minute. Slowly whisk in milk and broth until smooth; simmer until thickened, 3–4 minutes. Whisk in 1 tablespoon butter.
Off heat, stir in sour cream, 3/4 cup Parmesan, half the bacon, salt, and pepper.
Add green beans to the skillet and toss to coat. Spread the mixture in the prepared baking dish.
Combine panko, fried onions, remaining 1/4 cup Parmesan, and 1 tablespoon melted butter. Sprinkle evenly over the casserole.
Bake until bubbly and golden on top, 20–25 minutes. Rest 5 minutes, then scatter remaining bacon over the top and serve.