Clean and wash the spinach. Put it to boil in salted water, drain and chop it finely.
Fry the flour in heated butter. Top it up with milk and 1 cup of the water, in which you boiled the spinach. Allow the sauce to boil for 5 minutes over low heat, then remove it from the heat and thicken it with 3 beaten egg yolks. Whip the whites into snow with a pinch of salt.
Add the spinach, salt and egg whites and mix gently. Pour the mixture into a buttered tray, sprinkled with breadcrumbs. Grate cheese and distribute butter cubes on top. Bake the spinach soufflé in a heated oven and serve it immediately.