Heat the oven to 425°F.
Using a sharp knife, cut a pocket in each of the 4 chicken breasts, being careful to leave about a 1/4 inch around the edge so you don't accidently slice through the other side. Set the chicken breasts aside to come to room temperature.
Heat the olive oil in a medium skillet over medium-high heat. Add the onions and lightly season with salt and pepper. Saute until they are soft, about 5 minutes.
Add the spinach and parsley and allow it to wilt, stirring often, about 5 minutes.
Next, pour in the white wine and let the liquid cook off. The spinach and onions will absorb the wine and flavor.
Transfer the spinach and onions to a mixing bowl and add the kalamata olives, basil, and feta cheese. Lightly season with salt and pepper.
Divide the mix into 4 equal parts. Stuff each chicken breast with the spinach mix. Use toothpicks to close the pocket.
Mix the spices - paprika, salt, oregano, dried parsley, garlic powder, onion powder, and black pepper - in a small bowl. Sprinkle some of the spice mix on both sides of each chicken breast. Rub and press the spices into the meat.
Heat 2 tablespoons olive oil in a medium skillet over medium-high heat. If you have a cast iron skillet large enough, this works the best as this can take the high heat of the oven.
Place the chicken breasts in the skillet and cook 2 minutes on each side.
Place the skillet in the oven and cook for 15-18 minutes, or until the chicken reads 165°F on an instant read meat thermometer. When you remove the chicken, baste each piece with the juices in the skillet to prevent them from drying out.
While the chicken is cooking, make the Balsamic Vinaigrette.
In a medium or large mixing bowl, add in the balsamic vinegar, oregano, basil, parsley, garlic powder, salt, and pepper.
Slowly drizzle in the olive oil while whisking the ingredients together. This will thicken the oil to make the dressing.
Remove the toothpicks and baste the chicken on more time just before serving. Serve the chicken with the vinaigrette and fresh chopped parsley.