Transfer the salmon to a plate and set aside. Stir the crème fraîche into the leek mixture then slowly add the reserved pasta water. Bring to a bubble and season to taste with salt and pepper.
Put water on to boil for the pasta and, when it’s boiling ... remove them to a cold plate. Now stir the butter and sugar together in the frying pan until you have a hot, treacly syrup.
Scientists at University College London created what they are calling the "world's thinnest spaghetti." Don't expect to see this item on restaurant menus any time soon, however.