Set large pan over medium heat. Drizzle in olive oil. Once the olive oil begins to shimmer, add the chicken pieces and the red pepper and cook, stirring frequently, until the chicken is cooked throughout, about 8 to 10 minutes.
Stir in zucchini, tomatoes, pesto, and mozzarella cheese, if using.
Notes
To make ahead. Prepare according to the instructions. Allow to cool and store in an airtight container in the refrigerator for up to 4 days. Reheat to serve.To freeze. Prepare the chicken and pepper according to the directions. Hold the remaining ingredients to add and cook before serving. Store in the freezer for up to 3 months. Thaw in the refrigerator overnight. Add the remaining ingredients according to the recipe directions and serve.