Steam the Spinach in the microwave for 2-3 minutes.

In a mixing bowl, combine the steamed spinach, ½ cup of shredded mozzarella cheese, and grated Parmesan cheese. Mix well to create the stuffing.

Using a sharp knife, make a horizontal slit along the side of each chicken breast to create a pocket for the stuffing. Be careful not to cut all the way through.

Stuff each chicken breast with the spinach and cheese mixture, pressing gently to ensure the filling is evenly distributed.

Add two slices of the tomato to each chicken breast and top with remaining mozzarella cheese.

If using toothpicks, secure the tomatoes and open sides of the chicken breasts with toothpicks to keep the stuffing from falling out during cooking. Alternatively, you can use kitchen twine to tie the chicken breasts closed.

Transfer the pan to the prepared dish and bake for about 20-25 minutes or until the chicken is cooked through and reaches an internal temperature of 165°F (74°C).