Preheat the oven to 300F.
Add 1 tbsp of olive oil to a skillet over medium heat. Add in the spinach and sauté until just wilted, about 2-3 minutes.
When the spinach is finished cooking, press out the excess water with a paper towel or dish towel. Chop the spinach into smaller chunks.
To a medium-sized bowl, add the spinach chunks, pesto, sun-dried tomatoes, pine nuts, and feta cheese. Mix until combined.
Carefully slice a pocket into each salmon filet, being sure not to get too close to the edges.
Fill each pocket with one-fourth of the stuffing mixture.
Sprinkle each filet with salt and pepper.
Arrange salmon filets on a parchment-lined baking sheet. Cook in the oven for 20-22 minutes, or until the salmon has cooked through.