In a mixing bowl, combine the chopped spinach, cream cheese, feta cheese, minced garlic, lemon juice, black pepper, and salt. Mix until well combined.

Preheat the oven to 375°F (190°C).

Place the salmon fillets on a baking dish. Using a sharp knife, make a pocket in each fillet by slicing it horizontally, being careful not to cut all the way through.

Stuff each salmon fillet with the spinach mixture, pressing gently to secure the stuffing.

Drizzle olive oil over the stuffed salmon and place lemon slices on top.

Bake in the preheated oven for 20-25 minutes or until the salmon is cooked through and flakes easily with a fork.