Preheat the oven to 350°F.

In a small bowl, combine the spinach, feta, yogurt, green onions, garlic, and salt and pepper to taste.

Use a sharp paring knife to cut a 1-inch-wide slit into the thick end of each chicken breast and carefully cut down to the center to form a pocket, being extra careful not to slice all the way through. Divide the stuffing mixture among the pockets. Press to seal shut, using a toothpick if necessary. Sprinkle the chicken with 1/2 teaspoon salt and 1/4 teaspoon pepper.

Heat the oil in a large oven-safe skillet over medium-high heat. Add the chicken and sauté for 4 minutes, or until golden. Turn the chicken over and place the skillet in the oven. Bake for 12 minutes, or until the chicken reaches an internal temperature of 165°F. Let rest for 5 minutes before serving.