In a skillet, heat olive oil over medium heat. Add minced garlic and sauté for about 1 minute until fragrant.
Add the chopped spinach and cook until wilted, about 3-4 minutes. Remove from heat and let cool slightly.
In a bowl, combine the ricotta cheese, half of the mozzarella, and half of the Parmesan cheese with the wilted spinach. Mix well and season with oregano, salt, and pepper.
Season the chicken breasts with salt and pepper. Use a sharp knife to create a pocket in each chicken breast, being careful not to cut through completely.
Stuff each chicken breast with the spinach and cheese mixture.
In a baking dish, arrange the stuffed chicken breasts. Squeeze fresh lemon juice over them and sprinkle remaining mozzarella and Parmesan on top.
Bake in the preheated oven for 25-30 minutes or until the chicken is cooked through and the cheese is bubbly and golden.
Remove from the oven and let rest for a few minutes. Garnish with fresh basil leaves before serving.