Heat 1 tablespoon of olive oil in a skillet over medium heat. Add 1 sliced onion and sauté for 3–4 minutes until translucent. Stir in 2 minced garlic cloves and cook for an additional 30 seconds. Remove from heat.

In the skillet, stir in 2 cups of thawed and drained spinach, cooking for 2–3 minutes until heated through. Season with salt and pepper to taste, then mix in ½ cup of sour cream until fully coated.

Preheat oven to 175°C (347°F). Season salmon fillet with salt and pepper. In a non-stick skillet, sear salmon skin-side down for about 2 minutes until golden brown, then flip and sear the other side briefly.

Grease a baking dish with olive oil and arrange the seared salmon skin-side down. Spoon the creamy spinach mixture over the salmon and sprinkle with 1 cup of shredded mozzarella.

Transfer dish to the preheated oven and bake for 15–20 minutes until the salmon is opaque and mozzarella is golden and bubbly.

Let the dish rest for a few minutes before serving. Spoon extra creamy spinach on the side.