If you prefer frying the eggs, melt the butter in a nonstick skillet over medium-high heat until the foam subsides, then slip in the eggs, cover and cook until the whites are set and the yolks are still runny, about 3 minutes. Smear the crepes with some of the mustard sauce, scatter the cheese on top and then top with ham. Fold each crepe in half and top with a fried egg, some salt, pepper, sauce and chopped chives.