Preheat the oven to 375F and cook the pasta according to package directions. Using a cast-iron skillet or any other ovenproof skillet cook the bacon until brown. Remove the bacon from the skillet and place it on a plate with paper towels.
Add the minced garlic to the same pan and cook for about a minute being careful not to burn it
Mix in the flour until it is well combined with the garlic. Carefully and gradually, whisk in the milk and beer into the flour mixture
Cook the sauce for about 2-3 minutes until it thickens and coats the back of a spoon
Take the skillet off the heat and add the Parmesan and Cheddar cheeses to the sauce. Stir in half of the bacon
Take the green onions and the drained elbow pasta and mix them into the sauce as well. Bake uncovered, for 15 minutes until bubbly. Serve with reserved bacon and extra green onions on top.