In 2-quart saucepan, heat 2 tablespoons butter over medium heat. Add flour; cook and stir 1 minute. Beat in milk, salt and pepper with whisk; heat to boiling. Reduce heat to medium-low; cook and stir 1 to 2 minutes or until thickened. Remove from heat; beat in Gruyère and Parmesan cheeses. Add cooked pasta, chicken, bacon and browned onions; gently stir to combine. Transfer to same 7-inch cast-iron skillet.