Preheat oven to 350°F (180°C). Spray 1-qt. (1 L) shallow casserole with no-stick cooking spray; set aside.
Heat olive oil in large skillet over medium-high heat and brown chicken, stirring occasionally. Stir in red peppers, onion and garlic, then Mexican Spice Mix. Cook, stirring occasionally, until vegetables are tender, about 3 minutes. Stir in 1/2 of the diced tomatoes and cook, stirring occasionally, until chicken is completely cooked, about 5 minutes. Let cool slightly.