For the chutney, heat the oil in a pan. Add the onion and fry for a few minutes until softened, but not coloured.
Add the remaining chutney ingredients and simmer for 5-6 minutes until the plums are soft and the chutney has reduced to a thick consistency.
Leave to cool while you prepare the cheese on toast.
For the cheese on toast, preheat the grill to hot. Mix the cheese, mustard and egg yolk together in a small mixing bowl. Spread the mixture over the slices of bread.
Place onto a baking sheet and cook under the grill for 4-5 minutes until the cheese has melted and is golden-brown.
Slice the toast in half and arrange on a plate. Serve with the cranberry and plum chutney.