Heat olive oil in a skillet over medium heat. Add sliced onion and sauté for 3-4 minutes until translucent and fragrant. Add minced garlic and cook for another 30 seconds.

Incorporate thawed and well-drained spinach into the skillet. Cook for 2-3 minutes. Season with salt and pepper, then fold in sour cream, mixing until creamy.

Preheat oven to 175°C (347°F). Season salmon fillets with salt and pepper. Sear each side in a hot skillet for about 2 minutes until golden, careful not to cook through.

Lightly grease a baking dish and place salmon fillets skin-side down. Spread creamy spinach mixture over the salmon, then sprinkle with mozzarella cheese.

Transfer to the oven and bake for 15-20 minutes until salmon is opaque and flakes easily, and mozzarella is golden and bubbly.

Let the salmon rest for a couple of minutes before serving. Pair with the creamy spinach on the plate.