01 -
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.
02 -
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.
03 -
Return skillet to medium heat. Add mushrooms, onion, and garlic. Cook the vegetables until they are browned, about 5 minutes, stirring occasionally.
04 -
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.
05 -
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.
06 -
Once the cheese is completely melted into the sauce, add green beans and half of the cooked bacon.
07 -
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.