In a skillet, heat 2 tablespoons of olive oil over medium heat. Add 1 diced onion and sauté for 3-4 minutes until it becomes translucent, releasing a fragrant aroma. Stir in 2 minced garlic cloves, cooking for an additional 30 seconds.

To the aromatic mixture, stir in 4 cups of thawed and drained spinach. Cook for 2-3 minutes, allowing it to heat through. Season with salt and pepper to taste, then mix in ½ cup of sour cream, creating a creamy blend.

Preheat your oven to 175°C (347°F). Season 4 salmon fillets with salt and pepper on both sides. In a hot skillet, sear the fillets for about 2 minutes on each side until golden brown but slightly raw in the center.

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

Transfer to the preheated oven and bake for 15-20 minutes until salmon flakes easily with a fork and mozzarella is bubbly and golden brown.

Once baked, remove from the oven and let it rest briefly before serving each piece of salmon with a portion of the creamy spinach mixture beside it.