Heat olive oil in a skillet over medium heat. Add sliced onion, sautéing until translucent and soft (about 3-4 minutes). Stir in minced garlic and cook until fragrant (roughly 30 seconds).

Gently stir in thawed spinach. Cook for 2-3 minutes until heated through, mixing in salt and pepper to taste. Fold in sour cream for a creamy texture.

Preheat your oven to 175°C (347°F). Season salmon fillets with salt and pepper. Quick sear each side of the fillets for about 2 minutes in the same skillet.

Grease a baking dish lightly with olive oil, place seared salmon skin-side down, and spoon creamy spinach mixture over each fillet. Sprinkle grated mozzarella cheese on top.

Transfer to the oven and bake for 15-20 minutes. Ensure salmon turns opaque and flakes easily. Check that the internal temperature reaches 63°C (145°F).

Let the salmon rest for a few minutes before serving. Garnish with fresh herbs like parsley or dill if desired.