2 slices good bread
Sliced gruyere cheese
Thinly sliced, aged ham (proscuitto works great)
Spread mustard on both slices of bread. In a large frying pan, melt the butter, and place the bread in it, mustard-side up. On one half, arrange the sliced cheese, and on the other, the ham. Cook until the bread is nice and brown on the bottoms, and flip the cheese half onto the ham half to assemble the sandwich, keeping it on one side of the pan. Crack the eggs into the empty side of the frying pan and fry until the white is mostly cooked. Cover the pan with a lid to finish melting the cheese on the sandwich, and to just cook the top of the egg yolk (you want the yolk to still be runny, just not raw). Serve the eggs on top of the sandwich, with a nice green salad on the side.