Bring a large pot of salted water to a boil. Add green beans and blanch for 3-4 minutes until bright green and slightly tender. Drain and rinse with cold water to stop the cooking process. Set aside.
In a large skillet over medium heat, cook bacon until crisp. Remove, crumble and set aside. Drain excess fat, leaving 2 tablespoons in the skillet.
Add butter to the skillet. Saute diced onion until translucent, about 4-5 minutes. Add sliced mushrooms and cook until browned and moisture evaporates, about 5-6 minutes. Stir in minced garlic and cook for 1 minute more.
Sprinkle flour over vegetables and stir well. Cook and stir for 2 minutes. Slowly whisk in milk and broth, stirring constantly until the sauce thickens, about 3-5 minutes.
Stir in cheese, Worcestershire sauce, salt and pepper. Taste and adjust seasoning if necessary.
Gently stir in the green beans and bacon into the sauce. Mix until well combined.
Transfer the mixture to a greased 9x13-inch baking dish. Top evenly with 1 cup of the fried onions.
Bake in a 350 degree oven for for 25-30 minutes, until bubbly. Remove from the oven, sprinkle remaining fried onions on top, and bake for another 5 minutes until golden and crispy.