Heat one tablespoon of avocado oil in a large nonstick skillet over medium-high heat. Add the onion and sauté for 2 to 3 minutes, until very fragrant.

Stir in the mushrooms and ½ teaspoon of sea salt. Cover the skillet and cook for 3 minutes, stirring occasionally, or until much liquid is seeping out of the mushrooms. Add in the garlic and continue cooking until most of the liquid has evaporated.

Add in the baby spinach, cover the skillet and cook, stirring occasionally, until the spinach has wilted.

Stir in the feta cheese and stir for another 1 to 2 minutes, until the cheese is well incorporated.

Preheat the oven to 350 degrees F.

Line a large rimmed baking sheet with parchment paper. Place the salmon on a cutting board and pat salmon fillets dry with a paper towel and transfer them to the prepared baking sheet.

Slice the salmon through the center carefully using a sharp knife, leaving about ¼ to ½ inch at the end, creating a flap.

Stuff the salmon filets with the spinach and mushroom stuffing mixture.

Coat the flesh of the salmon with avocado oil using a silicon brush and sprinkle with sea salt and black pepper.

Bake on the center rack of the preheated oven for 25 to 30 minutes, or until the salmon is cooked through. Salmon is considered fully cooked once it has reached an internal temperature of 145 degrees Fahrenheit. Insert an instant-read thermometer into the thickest part of the salmon in order to get an accurate temperature read.

Garnish the stuffed salmon filets with fresh parsley or chopped green onion if desired. Serve stuffed sockeye salmon with your choice of favorite sides and enjoy!