¼teaspoonfreshly ground black pepper, more to taste
1cupshredded sharp white cheddar cheese
3cupsfrench fried onions, or to taste
Preheat oven to 350ºF. Spray a 9x13-inch pan with nonstick cooking spray.
Place cooked green beans in a large bowl.
In a deep sauté pan, cook bacon over medium heat until crispy. Remove to a paper-towel lined plate, reserving the grease in the pan.
Over medium heat, sauté the onions in the bacon grease for 2 to 3 minutes, or until starting to become translucent. Add mushrooms and continue to sauté until cooked. Use a slotted spoon to remove mushrooms and onions from the pan and put them in the bowl with the green beans.
Make sauce: Add butter and garlic to the pan (with any juices remaining in pan). Heat over medium heat until butter is melted and garlic is fragrant. Add flour and whisk for 1 to 2 minutes. While whisking, pour in milk and continue to cook, whisking nearly constantly, until mixture is thickened and bubbly. Add thyme and pepper, whisk to combine. Remove pan from heat and stir in cheese until melted.
Pour cheese sauce mixture over the green beans, mushrooms, and onions in the bowl. Add cooked bacon and stir until everything is well coated.
Pour the green bean mixture into the prepared pan, spreading it out in an even layer. Sprinkle on french fried onions.
Bake for 20 minutes, or until casserole is heated through and onions are golden brown.
Any type of green bean can be used, fresh or frozen. Canned green beans are fine, too, and don't need to be precooked before adding. Drain well. Use a comparable amount.
You may use fewer (or no) fried onions if you want.
To prep ahead, prepare the entire casserole, omitting onions, and refrigerate unbaked. Add onions prior to baking and bake as directed, adding 10 minutes baking time.