Preheat oven to 400 F.
Saute veggies. Add olive oil (1 tablespoon) to a large pan over medium heat. Once shimmering, add diced yellow onion (1/2 cup) and sauté until translucent, about 10 minutes. Add fresh spinach (6 Ounces) (you may need to do this in batches) and toss as it cooks. Once the spinach has wilted and has released its water, add minced garlic (2 cloves) and sauté for 1-2 minutes, or until the garlic is fragrant.
Combine filling. Add veggies to a large bowl and combine with crumbled feta ( 4 Ounces), julienned sun dried tomatoes (1/2 cup), salt (1 teaspoon), black pepper (1/2 teaspoon), oregano (1 teaspoon), and basil (1 teaspoon). Set bowl aside.
Prep chicken. Lay chicken breast (4 boneless, skinless) flat on a cutting board. Dry both sides of the chicken with a paper towel. Carefully butterfly each chicken breast, or create a pocket in the chicken breast.
Stuff chicken. Divide the filling into the chicken breasts and fold the breast over the filling to form a “pocket” (you can use toothpicks to keep everything together). Add bell pepper (1), olives (1/2 cup), and sliced lemon to the pan and brush the tops of the veggies and chicken with olive oil.
Bake in the oven for 20-24 minutes, or until the internal temperature of your chicken reaches 165 F. Allow the chicken to cool for 10 minutes before slicing, and enjoy!