In a large bowl, combine chicken, oil, Italian seasoning, garlic powder, and black pepper. Stir well to coat the chicken evenly with the seasonings.
Preheat the grill to medium-high heat. Grill the chicken for 6-8 minutes on each side, or until cooked through.
While the chicken is cooking, prepare the tomato basil topping. In a medium bowl, mix the diced tomatoes, salt, pepper, lemon juice, and chopped fresh basil.
Once the chicken is fully cooked, top each chicken breast with a slice of mozzarella cheese. Allow the cheese to melt for about 1 minute.
Finally, top the chicken with the tomato basil topping and serve immediately.