Preheat oven to 375°F (190°C).

In a skillet, heat olive oil over medium heat. Add chopped onion and garlic, and sauté until onion is translucent.

Add chopped spinach to the skillet, and cook until wilted. Remove from heat and let cool slightly.

In a bowl, mix together the cooked spinach mixture, feta cheese, and sun-dried tomatoes. Season with salt and pepper to taste.

Carefully butterfly each chicken breast and pound to even thickness.

Spoon the spinach and feta mixture onto each chicken breast and roll tightly, securing with toothpicks.

Place the stuffed chicken breasts in a baking dish, seam side down.

Bake in preheated oven for 25-30 minutes, or until chicken is cooked through and juices run clear. Garnish with your choice of herbs and sun-dried tomatoes (optional).