Prepare pasta according to package directions. Meanwhile, melt 2 Tbsp. butter in a Dutch oven over medium heat. Add onion and bell pepper; sauté 5 minutes or until tender. Add mushrooms; sauté 4 minutes. Remove from Dutch oven.
Melt remaining 3 Tbsp. butter in Dutch oven over low heat; whisk in flour until smooth. Cook, whisking constantly, 1 minute. Gradually whisk in chicken broth and milk; cook over medium heat, whisking constantly, 5 to 7 minutes or until thickened and bubbly.
Stir chicken, sautéed vegetables, and hot cooked pasta into sauce. Add cheese, pepper, and salt. Serve with desired toppings.