Combine, Hellmann's® or Best Foods® Light Mayonnaise, garlic, vinegar and oregano in small bowl; remove and set aside 1 tablespoon.
Season chicken, if desired, with salt and pepper. Rub both sides of chicken with 1 tablespoon Mayonnaise mixture and refrigerate 20 minutes. Toss tomatoes, peppers and onion with remaining Mayonnaise mixture.
Heat large nonstick skillet over medium-high heat and cook chicken, turning once, until golden, about 4 minutes. Remove chicken and set aside.