Preheat the oven to 375°F (190°C). Rinse the chicken breasts and pat dry with paper towels.
On a cutting board, slice a pocket into the thick side of each chicken breast with a sharp knife, taking care not to cut through.
Rub the outside of each breast with 1 tablespoon avocado oil, then season evenly with the paprika, garlic powder, and sea salt. Arrange the breasts in a large casserole dish.
Heat the remaining 1 tablespoon avocado oil in a large skillet over medium-high heat. Add the finely chopped onion and sauté, stirring occasionally, until softened, about 5 minutes.
Add the chopped artichoke hearts, sun-dried tomatoes, and chopped kalamata olives to the skillet, then add the baby spinach. Cover briefly until the spinach wilts, about 1–2 minutes.
Remove the skillet from heat and stir in the feta cheese until everything is well combined; let the mixture cool slightly so it is easier to handle.
Spoon the stuffing into the pocket of each chicken breast, pressing gently to keep the filling inside.
Bake on the center rack for 35–45 minutes, or until the internal temperature of the chicken reaches 165°F (74°C).
Let the chicken rest for 5 minutes before serving.