Don't miss out on our special Christmas issue with free 2023 calendar - subscribe by 18 October!
Spread both slices of bread with the mustard. Top one slice with the cheese and prosciutto. Sprinkle the sage leaves over the other slice, then sandwich together. Butter the outsides and cook in a hot frying pan, weighed down by another heavy pan, for 2-3 mins on each side until the outside is crisp and the cheese has melted. Alternatively, cook in a sandwich toaster.