Heat 1 tablespoon of olive oil in a skillet over medium heat. Add 1 sliced onion and sauté for 3-4 minutes until they turn translucent and fragrant. Toss in 2 minced garlic cloves and cook for an additional 30 seconds.

Stir in 300 grams of thawed and drained spinach to the skillet with the aromatic mixture. Cook gently for 2-3 minutes, season with salt and pepper to taste, then fold in 200 grams of sour cream until well combined.

Preheat your oven to 175°C (347°F). Slice your salmon fillet into individual portions. Season each side with salt and pepper. Sear the salmon pieces for 2 minutes on each side, keeping the center slightly undercooked.

In a greased baking dish, arrange the seared salmon portions skin-side down. Spoon the creamy spinach mixture over each piece, then sprinkle 150 grams of mozzarella cheese evenly across the top.

Place the baking dish in the preheated oven and bake for 15-20 minutes. The salmon should turn opaque and flake easily, and the cheese should be golden.

Let the dish rest for a couple of minutes before serving. Pair with additional spinach or your favorite side dish.