Add the chopped spinach, grated Parmesan, cream cheese, minced garlic, salt, and pepper to a mixing bowl. Stir everything together until the mixture is creamy and evenly combined. Make sure there are no large clumps of cream cheese and that the spinach is well distributed throughout the filling.

Place each salmon fillet flat on a cutting board. Using a sharp knife, carefully slice a pocket into the thick side of each fillet, cutting lengthwise but stopping before you reach the other side. The pocket should be deep enough to hold the filling while keeping the fillet intact.

Divide the spinach Parmesan mixture evenly between the salmon fillets. Gently spoon the filling into each pocket, pressing it in lightly so it stays in place without overstuffing. Wipe away any excess filling from the outside of the fish.

Sprinkle the blackened seasoning generously over all sides of the salmon fillets. Use your hands to gently press the seasoning into the surface so it adheres well and forms a flavorful crust during cooking.

Heat a skillet over medium-high heat and add enough oil to lightly coat the bottom. Once the oil is hot and shimmering, carefully place the salmon fillets in the pan. Sear without moving them for a few minutes until a dark, well-formed crust develops on the bottom.

Gently flip the salmon and continue cooking on the other side. Reduce the heat slightly if needed and cook until the salmon is just cooked through and flakes easily with a fork. For thicker fillets, you can cover the pan briefly or finish cooking on lower heat to prevent burning.

Remove the salmon from the pan and let it rest for a couple of minutes before serving. This allows the juices to redistribute, keeping the salmon moist and tender. Serve warm while the filling is creamy and the exterior is perfectly crisp.