Season chicken breasts with salt and pepper. Place on a baking sheet and bake for 20 minutes or until fully cooked.
While the chicken is baking, prepare the guacamole. In a mixing bowl, mash the avocados. Add diced tomato, onion, minced garlic, lime juice, salt, and pepper. Mix well.
Once the chicken is cooked, remove from oven and top each piece with a generous amount of guacamole and shredded cheese.
Return to oven and bake for an additional 5 minutes or until the cheese is melted and bubbly.