In a skillet over medium heat, add olive oil, then toss in thinly sliced onions. Sauté for 3-4 minutes until they become translucent, stirring occasionally.

Next, add minced garlic and continue to sauté for an additional 30 seconds until fragrant.

Stir in the thawed spinach, cooking it for 2-3 minutes until heated through and well-blended with the onions and garlic.

Remove the skillet from heat, then mix in sour cream and season with salt and pepper to taste.

Preheat your oven to 175°C (347°F). While it's heating, season your salmon fillets with salt and pepper.

In a separate hot pan, add a touch of olive oil and briefly sear each fillet for about 2 minutes on each side.

In a greased baking dish, place the seared salmon fillets skin-side down.

Spread the creamy spinach mixture over each piece and sprinkle a generous amount of shredded mozzarella on top.

Place the baking dish in the preheated oven and bake for 15-20 minutes.

After baking, let the dish rest for 2 minutes before serving.