Preheat the oven to 375°F (190°C).
Place the 4 chicken breast halves in a single layer on a rimmed baking sheet or in a 9×13-inch baking dish.
In a medium mixing bowl, combine 1 cup mayonnaise, 1/4 tsp garlic salt, 1/2 cup grated Parmesan, 2 cups shredded cheddar, and the drained 7-oz jar of diced pimentos; stir until evenly blended.
Spread the pimento cheese mixture evenly over the top of each chicken breast.
Bake for 20 to 25 minutes, until the chicken is cooked through and juices run clear; an instant-read thermometer should register 165°F (74°C).
Optional: place under the broiler for 1–2 minutes to brown the cheese, watching closely to prevent burning.