Preheat the oven to 375°F (190°C). Rinse the chicken breasts and pat dry with paper towels.
Lay each breast on a cutting board and carefully cut a pocket horizontally through the thickest side, leaving the edges intact.
Rub the outside of each chicken breast with most of the avocado oil and season evenly with paprika, garlic powder, and sea salt; place the breasts in a large baking dish.
Heat 1 tablespoon of the remaining avocado oil in a large skillet over medium-high heat. Add the finely chopped onion and sauté about 5 minutes until soft.
Add the chopped artichoke hearts, sun-dried tomatoes, and chopped kalamata olives to the skillet, then stir in the baby spinach and cover for 1–2 minutes until the spinach wilts.
Remove the skillet from heat, stir in the crumbled feta until well combined, and let the mixture cool slightly so it is easy to handle.
Spoon the stuffing mixture into each chicken pocket, dividing it evenly among the four breasts.
Bake on the center rack of the preheated oven for 35 to 45 minutes, or until the chicken is cooked through and registers 165°F (74°C) internal temperature.
Let the chicken rest a few minutes before serving.