Preheat your oven to 400°F and line a baking sheet with parchment paper or lightly oil it. Pat the salmon fillets dry with paper towels—this helps them cook more evenly. Lay the fillets skin-side down on the prepared baking sheet. Using a sharp knife, carefully cut a pocket into each fillet by slicing lengthwise about three-quarters of the way through, being careful not to cut all the way through to the skin. This creates a pocket for your filling without separating the fillet into two pieces.

Heat 1 tablespoon of olive oil in a large skillet over medium heat. Add the thawed spinach (make sure it's squeezed very dry to avoid a watery filling) along with the Italian seasoning, paprika, cayenne, salt, and pepper. Cook for 2-3 minutes, stirring occasionally, until the spinach is heated through and any remaining moisture has evaporated. Remove from heat and stir in the crumbled feta, grated Parmesan, chopped roasted red peppers, and garlic powder until well combined. I like to let this mixture cool slightly before stuffing so it's easier to work with and won't wilt the raw salmon.

Brush the inside and outside of each salmon fillet with the remaining 1.5 tablespoons of olive oil, then season generously with salt and pepper. Divide the spinach-feta mixture evenly among the four fillets, spooning it into the pocket you created in Step 1. Don't overstuff—you want the filling to stay inside during baking, so pack it gently and leave a little space at the edges.

Place the baking sheet in the preheated 400°F oven and bake for 12-17 minutes, depending on the thickness of your fillets. The salmon is done when it flakes easily with a fork and reaches an internal temperature of 145°F. I check the thickest part of the fillet by gently pressing it—it should feel firm but still have a slight give. Avoid overcooking, as salmon can dry out quickly.