Prepare pasta according to package directions. Meanwhile, melt 2 Tbsp. butter in a Dutch oven over medium heat.
Add onion and bell pepper; saut 5 minutes or until tender.
Add mushrooms; saut 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, sauted vegetables, and hot cooked pasta into sauce.