In a cold large (12") cast iron skillet add bacon. Turn heat to medium and cook until crisp. Stirring occasionally. This will take about 25 minutes. See the shortcut method if you want to make this quicker.
Remove bacon from the skillet with a slotted spoon and place on a paper towel lined plate. Set aside. Pour bacon drippings into a jar to save for later use and return 4 tablespoons bacon drippings to the pan.
Return skillet to medium heat. Add mushrooms, onion and garlic. Cook the vegetables until they are browned, about 5 minutes, stirring occasionally.
Add 2 tablespoons bacon drippings, into the pan (or butter if you are out of bacon drippings). Add flour, salt, pepper, and red pepper flakes. Stir until the flour dissolves into the vegetables. Cook 1 minute to cook out the floury taste.
Add milk and stir to combine. Remove from the heat, to prevent the cheese from getting grainy. Add the cheese a handful at a time and stir between each addition.
Once the cheese is completely melted into the sauce add green beans and half of the cooked bacon.
Stir to combine, and return the skillet to medium heat. Continue to cook the casserole until the green beans are heated through, and the sauce is thickened, about 3-5 minutes. Top with remaining bacon.
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Notes
You can also use blanched fresh green beans, when available.Shortcut method: bake bacon in the oven until crisp. Cool and crumble. Set aside. Add 4 tablespoons butter to skillet. Once the butter is melted add mushrooms, onions and garlic. Continue with recipe as written above.Gluten-free instructions: Mix 1/4 cup of corn starch with 1/2 cup of milk in a resealable container. Seal and shake the living tar out of it to create a slurry. Omit the flour called for in the recipe. Add the remaining 1 1/2 cups of milk to the recipe. Cook until it starts to bubble. Add the slurry and stir to combine. Proceed with the recipe.