Preheat oven to 400°F (200°C) and lightly grease a rimmed baking sheet.

Heat olive oil in a skillet over medium heat; sauté garlic for 30 seconds, then add spinach and cook until wilted.

Transfer spinach to a colander and press firmly to remove all excess moisture, then mix with mozzarella and parmesan in a small bowl.

Using a sharp knife, cut a horizontal pocket into the side of each chicken breast, ensuring you do not cut all the way through.

Spoon the spinach and cheese mixture evenly into the chicken pockets and secure the edges with toothpicks.

Season the exterior of the chicken breasts with salt and pepper.

Prepare three shallow bowls: one with flour, one with the beaten eggs, and one with panko breadcrumbs.

Dredge each stuffed chicken breast in flour, dip in the egg wash, and then press into the panko until thoroughly coated.

Arrange the chicken on the prepared baking sheet and drizzle with melted butter.

Bake for 22-25 minutes or until the internal temperature reaches 165°F (74°C) and the crust is golden brown.

Remove toothpicks before serving.