Thinly slice the white leeks and brown them in butter for a long time until they turn russet.
Meanwhile, put the salmon steaks in the microwave for less than a minute so that you can crumble them easily.
Butter a dish and successively place a plate of lasagna on it, then over all the smoked salmon and again a plate of lasagna.
Stop cooking the leeks and place them on the last layer of lasagna and cover with lasagna.
Cover the layer of lasagna with the crumbled salmon, cover with béchamel and a last plate of lasagna.
On the last layer of lasagna, pour the remaining béchamel sauce, sprinkle with grated Gruyère cheese and bake for 30 minutes in a medium / hot oven (thermostat 6/7).