Heat oven to 350°F. Spray 13x9-inch (3-quart) glass baking dish with cooking spray; set aside. Cook and drain pasta as directed on package, using minimum cooking time.
In 12-inch nonstick skillet, heat 1/4 cup butter over medium-high heat. Add onions, thyme, salt and pepper. Cook 11 to 12 minutes, stirring occasionally, until onions are tender. Reduce heat to medium; cook 10 to 12 minutes, stirring occasionally, until onions are starting to brown.
Sprinkle flour over onions in skillet; cook and stir about 1 minute. Stir in broth; heat to boiling, stirring constantly. Simmer 1 to 2 minutes or until slightly thickened. Remove from heat; stir in chicken and cooked pasta, mixing to combine.
Add half of the mixture to baking dish. Sprinkle with half of the cheese. Top with remaining pasta mixture, then remaining cheese. Cover and bake 20 minutes.
In small bowl, mix Topping ingredients. Remove cover; sprinkle on top of pasta mixture; bake 5 to 10 minutes or until topping is golden brown and mixture is heated through.
Notes
Try substituting shredded mozzarella cheese for the Swiss cheese, if desired.
Leftover cooked chicken also works in place of rotisserie chicken.