Heat olive oil in a skillet over medium heat. Add sliced onions and sauté for 3-4 minutes until translucent, stirring frequently. Add minced garlic and cook for 30 seconds until fragrant.

Add thawed and well-drained spinach to the skillet. Cook for 2-3 minutes, stirring gently until heated through and moisture evaporates. Season with salt and pepper, remove from heat, and stir in sour cream until creamy and combined.

Preheat oven to 350°F (175°C). Season salmon fillets generously with salt and pepper on both sides.

In a separate pan over medium heat, sear salmon for about 2 minutes per side to lock in moisture. The salmon should be slightly undercooked as it will finish in the oven.

Place seared salmon skin-side down in a greased baking dish. Spread the creamy spinach mixture evenly over the top of each fillet, then sprinkle generously with mozzarella cheese.

Bake in preheated oven for 15-20 minutes until salmon reaches 145°F internally and flakes easily with a fork. The cheese should be golden and bubbly.

Remove from oven and let rest for 2-3 minutes before serving. Enjoy your perfectly cooked baked salmon with creamy spinach!