Heat 2 tablespoons of olive oil in a skillet over medium heat. Add sliced onions and sauté for 3-4 minutes until they become translucent and fragrant. Then, incorporate minced garlic, stirring for another 30 seconds until it's golden and aromatic.

Stir in thawed and well-drained frozen spinach into the skillet, continuing to heat through for 2-3 minutes. Season generously with salt and pepper to taste, mixing well. Finally, add 1/3 cup of sour cream, ensuring it's evenly distributed.

Preheat your oven to 175°C (347°F). Season the salmon fillets with salt and pepper on both sides. Sear each fillet skin-side down in the same skillet for about 2 minutes.

Lightly grease a baking dish with olive oil. Place the seared salmon skin-side down in the dish, and spoon the creamy spinach mixture generously over the top of each fillet. Sprinkle mozzarella cheese evenly over the spinach layer.

Transfer the dish to the oven and bake for 15-20 minutes. The salmon should turn opaque and flake easily when done.

Remove the dish from the oven and allow it to rest for a few minutes before serving. Garnish with fresh herbs, if desired.