Preheat the oven to 375°F (190°C). Gather your ingredients and tools.

In a large mixing bowl, combine spinach, cream cheese, mozzarella, Parmesan, garlic powder, salt, and pepper. Mix until well combined.

Slice the salmon fillets lengthwise to create pockets, being careful not to cut through completely.

Stuff each salmon pocket with the creamy spinach mixture, packing it tightly.

Heat olive oil in an oven-safe skillet over medium heat. Place the stuffed salmon seam-side down and sear for 2-3 minutes until golden brown.

Flip the salmon and sear the other side for another 2-3 minutes. Then, transfer skillet to the oven and bake for 20 minutes, until salmon flakes easily.

Let the salmon rest for a couple of minutes before serving warm, garnished with extra Parmesan if desired.