1 Heat oven to 425°F. In 7-inch cast-iron skillet, heat 1 tablespoon of the butter over medium heat. Add onion; cook 8 to 10 minutes, stirring occasionally, until onions are browned. Remove from heat.
Meanwhile, cook pasta as directed on package to al dente; drain.
3In 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.
4In small bowl, mix bread crumbs and 1 tablespoon melted butter. Sprinkle over top of pasta mixture. Bake 14 to 17 minutes or until bubbly and golden brown.