Preheat oven to 375 degrees.
Place green beans in a microwave safe dish and add ⅔ cup water. Cover and microwave to lightly steam for 6 minutes.
While beans are cooking, cook bacon in an over safe skillet over medium heat(cast iron works well) till crisp. Remove from pan and set aside, but be sure to keep the dripping in the skillet.
Add flour to the pan with the drippings and whisk. Add the milk, salt, and pepper. Whisk and allow the mixture to thicken (about 3-4 minutes).
Whisk in the cheddar cheese. Remove beans from the water and add to the cheese sauce. Stir to evenly coat the beans in the cheese sauce. Top with Parmesan cheese and then the chopped cooked bacon.
Bake uncovered for 20 minutes.