In a skillet over medium heat, add a tablespoon of olive oil and heat until shimmering. Toss in sliced onions and sauté for about 3-4 minutes, stirring frequently until they turn translucent. Add minced garlic and cook for an additional 30 seconds until fragrant.

Next, incorporate the thawed and drained spinach into the skillet. Stir gently while cooking for about 2-3 minutes, until heated through and the moisture evaporates. Season the spinach mixture with salt and pepper to taste, then remove the skillet from heat and mix in sour cream, stirring until creamy and well combined.

Preheat your oven to 175°C (347°F). While the oven preheats, slice the fresh salmon into two portions and season each piece generously with salt and pepper. In a separate pan over medium heat, briefly sear each side of the salmon for about 2 minutes to lock in moisture.

In a greased baking dish, place the salmon skin-side down. Evenly spread the creamy spinach mixture over the top of the salmon, then sprinkle a generous layer of mozzarella cheese over the spinach.

Transfer the assembled dish to the preheated oven and bake for 15-20 minutes until the salmon turns opaque and flake easily when cooked through, and the cheese should be golden and bubbly.

Once baked, remove the dish from the oven and let it rest for a couple of minutes. Plate the baked salmon with a generous serving of creamy spinach alongside.