Preheat your oven to 375°F (190°C), ensuring the rack is in the center.

Heat a splash of olive oil in a skillet over medium heat. Sauté the minced garlic and chopped onion until aromatic and lightly golden, about 3-4 minutes.

Add the chopped spinach to the skillet. Cook, stirring occasionally, until the spinach wilts down, then remove from heat and let it cool.

Combine the wilted spinach with cream cheese, grated parmesan, shredded mozzarella, lemon zest, and freshly squeezed lemon juice in a mixing bowl. Stir until well incorporated.

Create a pocket in each salmon fillet by gently slicing down the side, ensuring not to cut all the way through. Stuff each pocket generously with the creamy spinach mixture.

Season both sides of the salmon fillets with salt and pepper. In an oven-safe skillet, sear the salmon skin-side down for about 2-3 minutes until golden and crispy.

Bake the salmon in the preheated oven for 12-15 minutes, or until the salmon is flaky and cooked through, reaching an internal temperature of 145°F (63°C).

Garnish with chopped parsley and lemon slices for a fresh finish before serving.