The greatest frustration with toasties is when the bread is burnt on the outside while the cheese inside has not melted. With the following approach you solve that problem. For the ultimate glossy crunch, bread that is nicely browned but not burnt, use mayonnaise instead of butter. It sounds crazy, but it really works! The oil in the mayonnaise gets hotter than butter and together with the egg it creates a nice, equal, crispy crust.
Preheat the oven to 180 °C. Place the slices of bread on a bread board and spread a generous layer of mayonnaise over them. Turn four slices over and place cheese and ham on top. Slice the leek into very fine rings and put these on the toasties. Then place the other slices of bread on top, with the mayonnaise side upwards. Warm a frying pan and fry two toasties until they have nice brown crusts. Flip over and repeat. Do the same with other two toasties, while keeping the first two warm in the preheated oven. Serve with spicy mustard.
