1. Preheat the oven to 200°C (400°F).
2. Drizzle some olive oil in the bottom of the dish and place the 3 uncooked lasagna sheets on top (they will absorb the oil and become softer).
3. In a sauté pan or wok, add the frozen spinach and water, cover, and cook for 20 minutes, stirring regularly.
4. Once the spinach is cooked (completely thawed and soft), add three or four pinches of nutmeg, two turns of the pepper mill, and 2 pinches of salt (be careful, smoked salmon is salty, so don't add too much salt!) and mix well.
5. Spread a layer of spinach in the dish, then half of the ricotta using a tablespoon (spread evenly).
6. Add 200 g (7 oz) of sliced smoked salmon on top of the mixture and sprinkle with a little grated gruyère cheese.
7. Repeat the same layers by adding the second layer of pasta, spinach, remaining ricotta, and salmon.
8. Finally, add a last layer of spinach, sliced mozzarella, and generously sprinkle with gruyère cheese.
9. Bake for 25 minutes at 200°C (400°F).