In a large skillet over medium heat, warm 1 tablespoon of olive oil. Add the sliced onions and sauté for 3-4 minutes until they turn translucent and aromatic. Stir in 2 minced garlic cloves and sauté for another 30 seconds.

Incorporate 2 cups of thawed and well-drained spinach into the skillet. Cook for an additional 2-3 minutes, stirring frequently. Season generously with salt and pepper, and mix in ½ cup of sour cream until well combined.

Preheat your oven to 175°C (347°F). Season 4 salmon fillets with salt and pepper on both sides. In a separate skillet over medium-high heat, add more olive oil and sear the salmon for about 2 minutes on each side until golden brown.

Grease a baking dish with olive oil and place the seared salmon fillets inside. Distribute the creamy spinach mixture over each fillet, ensuring they are well covered. Sprinkle 1 cup of shredded mozzarella cheese atop the spinach.

Transfer the assembled dish to the preheated oven and bake for 15-20 minutes. The salmon should be opaque and flake easily with a fork.

Allow the dish to rest for a couple of minutes before serving. Serve alongside fresh salad or creamy mashed potatoes.