Place a rack in the center of the oven and preheat to 425°F. Line a large rimmed baking sheet with parchment paper.

Heat 2 teaspoons extra virgin olive oil in a medium nonstick skillet over medium-high heat.

Add a large handful of the baby spinach and cook, stirring often, until it begins to wilt. Stir in the minced garlic, 1/4 teaspoon kosher salt, 1/4 teaspoon ground black pepper, and a pinch of red pepper flakes; cook about 30 seconds. Continue adding the remaining spinach in large handfuls, stirring and letting each batch wilt before adding the next, until all the spinach is wilted.

Remove the skillet from the heat. Stir in the 1/4 cup reduced-fat cream cheese until it melts and combines with the spinach. Stir in the 1/4 cup shredded mozzarella and 1/4 cup grated Parmesan until the cheeses melt and the mixture is well combined. Let the filling sit briefly to cool so it’s easier to handle.

Place the four 6-ounce salmon fillets on a clean work surface and pat them dry. Using a small sharp knife, cut a lengthwise pocket in each fillet without cutting all the way through.

Transfer the fillets to the prepared baking sheet. Spoon the spinach-and-cheese mixture into each fillet pocket, dividing it into four equal portions (use the whole mixture).

Drizzle the remaining 2 teaspoons extra virgin olive oil over the fillets and brush or rub to coat. Sprinkle the fillets with the remaining 1/2 teaspoon kosher salt and the remaining 1/4 teaspoon ground black pepper.

Bake on the center rack until the salmon is opaque in the center and flakes easily with a fork, 8 to 10 minutes. Remove from the oven and serve.