Heat oven to 350.
Heat oil in a medium skillet. Add chicken to the pan and sprinkle with onion powder, salt and pepper. Cook for 3 minutes on each side, or until lightly browned.
Transfer chicken to a greased baking plate and summit each breast with avocado, tomato and cheese. Drizzle with lemon juice.
Bake for 10-12 minutes or until cheese is melted and chicken is done.