Sauté the spinach in a pan until it wilts and arrange half of the spinach on the bottom of an ovenproof dish. Chop the onions into crescents, the courgette into slices and the salmon into cubes. Add the spinach, salmon, courgette, crème fraîche and feta to the baking dish in layers. Top with grated mozzarella. Bake at 200°C using upper and lower heat for 15-20 min. until the cheese is golden and the fish is completely cooked.