Heat a splash of olive oil in a skillet over medium heat. Add sliced onions and sauté for about 3-4 minutes until they become translucent and golden. Toss in minced garlic and continue to sauté for another 30 seconds until fragrant.

In the same skillet, add the thawed and well-drained spinach. Cook for 2-3 minutes, stirring occasionally, until heated through. Season with salt and pepper to taste, then mix in sour cream until creamy.

Preheat your oven to 175°C (347°F) while seasoning the salmon fillets with salt and pepper. Sear each side of the salmon in a separate pan over medium-high heat for approximately 2 minutes.

In a greased baking dish, lay the salmon fillets skin-side down. Spread the creamy spinach mixture over the top of each fillet and sprinkle mozzarella cheese on top.

Carefully place the assembled dish in the preheated oven and bake for 15-20 minutes until the salmon is opaque and flakes easily with a fork.

Allow the dish to rest for a few moments before serving. Plate each fillet with a generous serving of the creamy spinach mixture on the side.