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 stir for about 30 seconds.

Incorporate 3 cups of thawed spinach into the skillet. Stir for 2-3 minutes and season with salt and pepper. Stir in ½ cup of sour cream until combined.

Preheat oven to 175°C (347°F). Pat the salmon dry and season with salt and pepper. Sear in a skillet with olive oil for 2 minutes on each side.

In a greased baking dish, place the seared salmon skin-side down. Spread the spinach mixture over it and sprinkle with mozzarella cheese.

Bake for 15-20 minutes until the salmon is opaque and flakes easily with a fork, and the cheese is bubbling and golden.

Let it rest for a couple of minutes, then serve alongside remaining creamy spinach.