Begin by heating 1 tablespoon of olive oil in a skillet over medium heat. Once the oil shimmers, add sliced onions and sauté until they soften and become translucent, about 3-4 minutes. Stir in minced garlic and sauté for an additional 30 seconds until fragrant.

Next, stir in the thawed and drained spinach to the skillet, cooking until heated through, which should take around 2-3 minutes. Season this mixture with salt and pepper to taste, then fold in the sour cream until everything is well combined.

Preheat your oven to 175°C (347°F). Take bright pink salmon fillets, season both sides generously with salt and pepper, and lightly sear each side in a separate pan over medium-high heat for about 2 minutes.

Grease a baking dish with a bit of olive oil to prevent sticking. Arrange the seared salmon fillets skin-side down in the dish, then generously spread the creamy spinach mixture over each fillet. Top each portion with mozzarella cheese for even coverage.

Place the assembled dish in the preheated oven and bake for 15-20 minutes until the salmon turns opaque and flakes easily with a fork.

Once removed from the oven, let the baked salmon rest for a few minutes. Garnish with fresh herbs, and serve with a light salad or your favorite sides.