Preheat the oven to 400°F (200°C) and lightly grease a 9x13 inch baking dish with olive oil.
In a medium mixing bowl, combine the chopped spinach, crumbled feta, and minced garlic. Stir until well combined.
Place each chicken breast on a flat surface. Using a sharp chef's knife, cut a horizontal slit into the side of the thickest part of the breast to create a pocket, being careful not to cut all the way through.
Stuff each pocket with approximately 2 to 3 tablespoons of the spinach and feta mixture. Secure the edges with toothpicks if necessary.
In a small ramekin, whisk together the olive oil, lemon juice, oregano, paprika, salt, and pepper.
Arrange the stuffed chicken breasts in the prepared baking dish. Brush the oil and herb mixture generously over both sides of the chicken.
Bake in the center rack of the oven for 25 to 30 minutes, or until the internal temperature reaches 165°F (74°C) when measured with a meat thermometer.
Remove from the oven and allow the chicken to rest for 5 minutes. Remove toothpicks before serving.