Blanch: First, bring a pot of salted water to a boil. Add green beans and cook for 4-5 minutes. Drain and place in ice water to stop cooking. Drain again.
Sauté: Next, melt butter in a large skillet over medium heat. Add mushrooms and onion. Cook for 5-6 minutes until soft. Add garlic and cook for 1 minute.
Thicken: Sprinkle flour over the mushrooms and stir for 1 minute.
Simmer: Then, slowly whisk in the broth, milk, soy sauce, salt, and pepper. Simmer for 3 minutes until thickened.
Combine: Remove from heat. Stir in the green beans and 1/2 cup of crispy onions.
Bake: Transfer to a baking dish. Top with remaining 1 cup of crispy onions. Bake at 375°F (190°C) for 15-20 minutes until bubbly and golden.