1. Cook pasta according to package directions.
2. While pasta is cooking, coat a large skillet with cooking spray and heat over medium-high. Season chicken with salt, pepper, and garlic. Saute until cooked through (4-5 minutes).
3. Remove chicken from skillet and set aside. Add broccoli, red pepper, and water to skillet. Cover and simmer over medium heat until vegetables are tender (5-6 minutes). Transfer to bowl with chicken.
4. In a small bowl, combine 1/4 cup of the milk with cornstarch, whisking until smooth. Add to skillet with remaining milk and bring to a boil, stirring constantly.
5. Reduce heat and stir in mustard. Stir in chicken mixture.
6. Toss cooked pasta with chicken mixture and serve immediately.