Heat 2 tablespoons of olive oil in a skillet over medium heat. Add 1 sliced onion and sauté for 3-4 minutes until translucent, stirring occasionally. Then, introduce 2 minced garlic cloves to the pan, cooking for an additional 30 seconds until fragrant.

Stir in 2 cups of thawed and drained spinach into the skillet, heating for 2-3 minutes. Season the mixture with salt and pepper to taste, then mix in ½ cup of sour cream until well incorporated.

Preheat your oven to 175°C (347°F) while you season the salmon fillets with salt and pepper. Sear each fillet in a bit of olive oil for 2 minutes per side.

Grease a baking dish with olive oil and place the salmon fillets skin-side down. Spread the creamy spinach mixture over each fillet, then sprinkle 1 cup of mozzarella cheese on top.

Transfer the dish to the preheated oven and bake for 15-20 minutes until the salmon is flaky and the cheese is golden brown.

Let the salmon rest for a couple of minutes before serving warm, paired with mashed potatoes or a salad.