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.
I had to jazz it up some because it was bland. In addition to salt and pepper I added garlic powder and onion powder. The sauce came out runny but that could've been my mistake. Will make it again if I kind find a way to thicken the sauce.
It's really nice to find recipes that are as quick and good as this one. The chicken can be canned if you are in a real hurry or grilled if you have an extra couple of minutes. Even the pickiest eaters in the family like this.