Add 1 tablespoon of avocado oil to a large non-stick skillet and heat to medium-high. Add the onion and stir well. Saute until the onion is very fragrant and begins to soften, about 3 to 5 minutes.

Add the chopped mushrooms and stir well. Cover the skillet with a lid and allow the onion and mushrooms to cook until the mushrooms begin to leach liquid, about 5 minutes. 

Remove the lid and stir in the garlic and spinach. Cook, stirring constantly until the spinach has wilted. Stir in the feta cheese until combined.

Preheat the oven to 375 degrees F.

Place the chicken breasts on a cutting board and use a sharp knife to carefully cut a slit (or “pocket”) into each breast.

Transfer the chicken breasts to an oiled or sprayed large casserole dish.

Drizzle each chicken breast with avocado oil, then sprinkle liberally with sea salt, paprika, garlic powder, and black pepper. Drizzle the chicken breasts again with a small amount of avocado oil and use your hands to spread around the oil and seasonings over the breasts.

Scoop the stuffing into each chicken breast, filling them up to your heart’s desire. 

If you have them, use toothpicks to secure the filling into the chicken breasts (no worries if you don’t have them).

Bake for 30 to 35 minutes, or until the chicken breasts are completely cooked through.

Serve with your choice of sides and enjoy!