Heat the olive oil in a small skillet over medium heat. Add the minced garlic and small red pepper pieces, sautéing for about 2 minutes until softened. Toss in the fresh spinach and cook just until it wilts. Remove from heat, let cool slightly, and drain any excess liquid.

In a medium mixing bowl, combine the softened cream cheese, crumbled feta, and the cooled spinach and pepper mixture. Stir vigorously until the mixture is uniform, creamy, and rich.

Place your 4 salmon fillets flat on a cutting board. Using a sharp paring knife, slice a horizontal pocket into the side of each fillet, cutting about three-quarters of the way through.

Using a spoon, generously stuff the creamy spinach and feta mixture into the pocket of each salmon fillet. Press the top down slightly to secure. Arrange the stuffed fillets in your light pink ceramic baking dish.

Preheat your oven to 400°F (200°C). Brush the top of each stuffed fillet generously with the melted butter. Sprinkle evenly with kosher salt and coarse black pepper. Bake for 12 to 15 minutes, or until the salmon is perfectly flaky and pink.

Switch your oven to the broil setting for the final 1 to 2 minutes of cooking. Watch closely so the black pepper crust does not burn. Remove from the oven when the feta mixture is golden and bubbly, then serve immediately.