In a large skillet, heat 2 tablespoons of olive oil over medium heat. Add diced onions and sauté for 3–4 minutes until they turn translucent. Then, mix in minced garlic and continue to sauté for another 30 seconds.

Next, stir in approximately 2 cups of fresh or thawed spinach into the skillet. Cook for 2–3 minutes until the spinach is heated throughout and any excess moisture evaporates. Season with salt and pepper to taste, then fold in 1/2 cup of sour cream.

Preheat your oven to 175°C (347°F). Season the salmon fillet generously with salt and pepper. Sear each side for about 2 minutes until slightly browned.

In a greased baking dish, lay the seared salmon skin-side down. Spoon the creamy spinach mixture over the top. Finally, sprinkle 1 cup of shredded mozzarella on top.

Transfer the assembled dish into the preheated oven and bake for 15–20 minutes until the salmon is opaque and mozzarella is golden and bubbly.

Remove the dish from the oven and let it rest for a few minutes before serving.