What you need:
4 thick slices of bread
shredded cheddar - about 75g
slices of Gruyere - enough to cover the top of the bread slices
3 Tbls of vollrahm (cream)
butter or olive oil
While the leeks are cooking, slice your bread and put on a baking sheet. Add a slice of Gruyere to the tops of each of the bread. Turn the broiler on. Or use a toaster if you have on.
Add about 3/4 of the shredded cheese and mix well. Take off the heat
Top the slices of bread with the leek mixture. Shred the rest of the cheese and add to the top.
Keep in the oven until the cheese has melted and started to brown a bit. Keep in more or less depending on how crispy you want the cheese to be.