Wash spinach thoroughly in tepid water and remove stems. Boil for about 2 minutes in a kettle of water. When leaves become soft, drain but save water. Finely chop the onion and brown it in margerine or butter until golden. Dust with flour. Pour on the spinach water. Stir. Add pressed spinach. Add milk and slightly beaten egg yolk. Stir. Just before serving, add finely grated cheese and a bit of garlic.