Heat 2 tablespoons of olive oil in a skillet over medium heat. Add 1 sliced onion and sauté for 3-4 minutes until translucent. Then, add 2 minced garlic cloves and cook for 30 seconds until fragrant.

Stir in 2 cups of thawed spinach and cook for 2-3 minutes until heated. Mix in ½ cup of sour cream and season with salt and pepper. Remove from heat.

Preheat oven to 175°C (347°F). Season 4 salmon fillets with salt and pepper. Sear each fillet for about 2 minutes on each side.

Grease a baking dish with olive oil. Place salmon skin-side down in the dish. Spread the spinach mixture over each fillet. Sprinkle 1 cup of mozzarella cheese on top.

Bake for 15-20 minutes until the salmon is opaque and flakes easily, and the cheese is golden and bubbly.

Let rest for a few minutes before serving. Plate the salmon with your favorite sides.