Heat 1 tablespoon of olive oil in a skillet over medium heat. Add 1 medium sliced onion and sauté until it turns translucent, about 3–4 minutes. Then, stir in 2 minced garlic cloves and cook for an additional minute until fragrant.

Incorporate 2 cups of thawed and drained spinach into the skillet, mixing thoroughly to allow it to heat through, roughly 2–3 minutes. Next, stir in 1/2 cup of sour cream and season with salt and pepper to taste.

Preheat your oven to 175°C (347°F). Season each 200g salmon fillet with salt and pepper. Heat another skillet, add a splash of olive oil, and sear the salmon fillets skin-side down for about 2 minutes until a golden crust forms. Flip and sear the other side for 1-2 minutes.

Grease a medium baking dish with olive oil and place the seared salmon skin-side down. Spread the creamy spinach mixture generously over each fillet. Sprinkle approximately 1 cup of shredded mozzarella cheese on top.

Bake the assembled dish in the preheated oven for 15-20 minutes until the salmon turns opaque and flakes easily with a fork, and the cheese becomes golden and bubbly.

Remove the dish from the oven and let it rest for a couple of minutes. Serve the salmon warm, garnished with any leftover spinach mixture.