Preheat your oven to 375°F (190°C).

Sauté onion and garlic in a skillet with olive oil over medium heat for 3-4 minutes until translucent.

Add chopped spinach to the skillet and stir until wilted, about 2-3 minutes. Remove from heat and let cool slightly.

Mix the cooled spinach with crumbled feta cheese and sun-dried tomatoes in a bowl, seasoning with salt and pepper.

Butterfly each chicken breast carefully, then pound to an even thickness.

Fill each chicken breast with the spinach and feta mixture, roll tightly, and secure with toothpicks.

Arrange stuffed chicken breasts seam side down in a greased baking dish.

Bake for 25-30 minutes until internal temperature reaches 165°F (74°C).