–1 kg (2 ⅕ lb) leeks - 200 g (7 oz) grated sharp cheddar cheese (or a mix of your favorite cheeses like Gruyère, Fontina, or Emmental) - 250 ml (1 cups) heavy cream - 3 garlic cloves - 1 tsp. mustard (Dijon or whole grain works best) - 75 g (2 ⅔ oz) unsalted butter - ¼ tsp. ground nutmeg - salt and pepper to taste - fresh parsley, finely chopped (for garnish)
Cut off the green part and the root part of the leek, then cut the onion lengthwise into two halves, wash the onion, cut diagonally into pieces about 2 cm long, place in a bowl, set aside.
In a large skillet over medium heat, saute the leeks in butter for 10 minutes, stirring. Add garlic, salt and pepper and cook for about 10 minutes more, stirring occasionally, set aside.
Heat the cream in a small saucepan, add the nutmeg and mustard, then add a quarter of the cheese and stir thoroughly until dissolved. Remove the finished mixture from the heat.
Place the leeks in a baking dish, pour the sauce over them and sprinkle with cheese. Bake at 180°C (350°F) for 20-25 minutes until golden.