Preheat oven to 350°F (175°C) and lightly coat a 9x13-inch baking dish with nonstick cooking spray.
In a large skillet over medium-low heat, cook bacon until crisp. Transfer to paper towels to drain, then crumble. Discard all but about 2 tbsp of the bacon grease, return pan to medium heat, add onions, and cook, stirring occasionally, about 5 minutes until translucent.
Add mushrooms and cook, stirring occasionally, 5 minutes until golden brown. Add garlic and cook 1 minute until fragrant. Stir in undiluted mushroom soup and crumbled bacon. Stir in milk and cheese until cheese melts. Season with salt and pepper to taste. Fold in green beans until well combined.
Pour mixture into prepared dish. Top with fried onions. Bake uncovered 30 minutes until hot and bubbly.