To make the mostarda sauce, boil down the apple juice, vegetable stock and sugar with the bay leaf for 10-12 minutes. Remove the bay leaf, mix in the mustard and lemon juice and add salt and pepper to taste. Remove from the hob and leave to cool.Thinly slice the honeydew melon and avocado. Heat the oil in a pan and fry the grill-cheese according to the instructions on the packet for about 3-4 minutes until lightly browned on both sides. Slice the ciabattas and drizzle the bottom half with 1 tbsp mostarda sauce. Arrange the melon, avocado and grill-cheese evenly on top and add the remaining sauce. Place the top half of the ciabatta on top and gently press down. Serve immediately.