This is a nice filling dish and my vegetarian guest tonight seem to love it, I had no left-overs. Pick your leeks well, only young leeks that do not have a woody core in the centre. The centre should be pale yellow tender leaves, unlike the picture. About one leek per person.
Split four leeks vertically and clean them well under running cold water, cut them in slices 2 inches long, then cook them in salted water until tender. In the meantime, in a non-stick or cast-iron pot, rub a split garlic clove on the bottom. On low heat, add a small cube of butter, a tsp olive oil and a tbsp flour. Stir with a whisk and add one cup of cold whole milk, let thicken, add a pinch of nutmeg, salt to taste. Turn the heat off and add 1/4 cup cream, a beaten egg while stirring with the whisk. Add to that 100gr of grated cheese, Swiss or Gruyère are best. After you have drained the leeks really well, add them to the sauce and pour into a gratin dish. Grate another 50 gr of cheese on top and bake for 30 minutes until golden.
Note: the cream is optional, it can be replaced by 2 small cubes of butter or left out completely.
If you like leeks, look up:https://thecookingfrog.wordpress.com/2010/06/27/leek-salad/ and https://thecookingfrog.wordpress.com/2010/06/15/tarte-aux-poireaux/