Bring a saucepan of water to a boil over medium-high heat. Add the green beans and cook for about 5 minutes. Drain and rinse under cold water. Set aside.
Cook the bacon in a large skillet over medium heat until crispy. Transfer to a paper towel-lined plate to drain.
Drain most of the bacon grease from the skillet, leaving about 1 tablespoon.
Add the mushrooms to the skillet and cook over medium-high heat for about 5 minutes, until golden brown. Add the garlic and cook for 1 minute, until fragrant. Remove the skillet from the heat.
Chop or crumble the cooked bacon and return it to the skillet.
Stir in the cream of mushroom soup, milk, salt, and pepper until combined.
Gently fold in the green beans and 2 cups of the French fried onions.
Transfer the mixture to a casserole dish and bake for 30 minutes.
Stir the casserole, then sprinkle the remaining ½ cup French fried onions evenly over the top. Return to the oven and bake until the onions are golden and crispy.
Notes
Store leftovers in an airtight container in the refrigerator.