1. Bring a large pot of salted water to a boil. Cook the lasagna sheets for 5 to 6 minutes in the water, stirring gently from time to time. Transfer them to a bowl of cold water.
Sauté the washed and trimmed spinach in 20 g of butter, season with salt and pepper. In a saucepan, pour the fish stock and cream, then add the cornstarch and mix it in. Bring to a boil to thicken the sauce. Finish with the remaining butter and adjust the seasoning.
Preheat the oven to 200°C (400°F).
Spread a little sauce on the bottom of the dish, then place two lasagna sheets. Cover with half of the spinach and drizzle with a little sauce. Place two more lasagna sheets, then the salmon cut in half lengthwise. Sprinkle tarragon leaves over the salmon, cover with sauce, and repeat the process. Finish with the remaining two sheets and sauce. Bake for 20 minutes. Enjoy right out of the oven!