Melt the butter in a saucepan or on a pan over very low heat. Remove from the heat immediately after melting. Allow to cool. In a bowl, combine flour, butter, egg yolk (use the egg white later), and a pinch of salt. Mix and knead thoroughly with a spoon, then by hand, until the dough is uniform. Add a little water if necessary to make the dough more pliable. If the dough is too sticky, lightly dust it with flour.

Melt the butter in a saucepan or on a pan over very low heat. Remove from the heat immediately after melting. Allow to cool. In a bowl, combine flour, butter, egg yolk (use the egg white later), and a pinch of salt. Mix and knead thoroughly with a spoon, then by hand, until the dough is uniform. Add a little water if necessary to make the dough more pliable. If the dough is too sticky, lightly dust it with flour.

Wash, dry, and slice the mushrooms. Peel and finely chop the onion. Heat the olive oil in a pan, then sauté the onion for a few minutes, stirring occasionally. Add mushrooms and a pinch of salt, and sauté for a few more minutes until the ingredients soften and the liquid evaporates. Set aside.

Grate the cheese or cut it finely. In a tall container, such as a blender jar, combine cream, milk, and egg white. Beat with a mixer or a kitchen whisk. Add half of the grated cheese and season with salt and pepper to taste. Mix with a spoon.

Evenly spread the onion and mushrooms on the baked base. Sprinkle with the remaining cheese. Pour the cream and milk mixture over the top. Bake for about 15-20 minutes at 180 degrees Celsius until the mixture thickens.

Serve hot or cold. Before serving, sprinkle with chopped chives and a few drops of lemon juice.