Heat olive oil in a medium skillet over medium heat until shimmering. Add sliced onions and cook for 3-4 minutes, stirring occasionally, until soft and translucent. Add minced garlic and stir constantly for 30 seconds until fragrant but not browned.

Add the thoroughly drained and squeezed spinach to the skillet, breaking up any clumps. Cook for 2-3 minutes, stirring frequently, until heated through and any moisture evaporates. Season with salt and pepper, then remove from heat and fold in sour cream until creamy and well combined.

Preheat oven to 175°C (347°F) and lightly grease a baking dish with olive oil. Season salmon fillets generously with salt and pepper on all sides. In a clean skillet over medium heat with a drizzle of olive oil, sear each fillet for 2 minutes per side, starting flesh-side first, to develop a golden crust.

Place salmon fillets skin-side down in the prepared baking dish (or flat if using skinless). Spread the creamy spinach mixture evenly over each fillet, then sprinkle generously with shredded mozzarella cheese, covering all the spinach.

Bake for 17-20 minutes, checking thinner fillets at 15 minutes. Use a meat thermometer to verify salmon has reached 63°C (145°F) at the thickest part. The salmon should be opaque and flake easily, and cheese should be golden and bubbling. Let rest for 2-3 minutes before serving.