Season the chicken with 3/4 teaspoon of the salt and the black pepper.
Combine the onion, Sazon seasoning mix and corn oil in a shallow glass dish. Place the chicken in the dish; turn to coat the pieces evenly. Cover the dish with plastic wrap; refrigerate for at least 4 hours or overnight.
Heat a large heavy-bottomed nonstick skillet over medium-high heat. Place the chicken, meaty side down, in the hot skillet; cook for 5 minutes without turning over. Reduce the heat to medium-low; cook for another 3 minutes. Turn the chicken over; cook for 3 minutes or until the chicken is cooked through and the internal temperature registers 170 degrees F on an instant-read thermometer inserted in the center. Remove the chicken to a serving platter; keep warm.
Add the cherry tomatoes to the skillet; cook for 2 to 3 minutes or until softened. Season with the remaining salt.
To serve, spoon tomatoes over chicken breasts on a platter. Makes 4 servings.