Preheat the oven to 350 degrees.

Trim the green beans and toss them in a pot of boiling water for 2-3 minutes. Drain and pour the green beans into a 9 x 13 pan. Set aside.

Cook the bacon for 3-4 minutes per side in a saute pan over medium heat Remove the bacon to a paper towel lined plate.

Remove all but 1 tablespoon of bacon grease from the pan. Return the pan with the bacon grease to medium heat and add in the onion, mushrooms, and garlic. Saute, stirring occasionally, for 4 minutes.

Combine the milk and cornstarch in a small bowl and whisk until combined. Slowly pour the milk into the saute pan while stirring constantly. Cook for 2-3 minutes or until the sauce has thickened, stirring constantly. Remove from heat and stir in butter, salt, pepper, and rosemary. Mix well.

Pour the sauce and vegetables over top of the green beans.

Bake for 15 minutes. Remove from oven and top with fried onions. Continue cooking for 15 additional minutes.

Serve hot and top with more fried onions if desired.