Preheat your oven to 350°F (175°C) and spray a 9x13-inch baking dish with cooking spray.

In a large skillet, cook the thick-cut bacon over medium-low heat until crispy, about 10-12 minutes. Remove bacon, crumble, and reserve 2 tablespoons of drippings.

In the same skillet, sauté the chopped yellow onion over medium heat until translucent, about 5 minutes. Add the button mushrooms and cook until golden brown for an additional 5-7 minutes. Stir in minced garlic and sauté for another minute.

Lower the heat, then stir in cream of mushroom soup, crumbled bacon, milk, and Monterey Jack cheese. Mix until the cheese melts and the mixture becomes creamy, seasoning with salt and pepper to taste.

Gently fold in the canned cut green beans (or blanched fresh ones) until thoroughly coated.

Pour the mixture into the prepared baking dish, top with French fried onions, and bake for 30 minutes until hot and bubbly.

Let the casserole rest for a few minutes before serving to allow flavors to meld.