Heat oven to 200C/180C fan/gas 6. Microwave the potatoes on High for 12-15 mins until tender. Meanwhile, steam or boil the broccoli for 3 mins, then drain well. When the potatoes have cooled a little, cut them in half lengthways and scoop the insides into a bowl.
Put the potato shells on a baking sheet. Mash the flesh with a fork, then stir in the mustard, egg, most of the cheese and the broccoli. Season if needed, then pile back into the shells. Sprinkle with the reserved cheese and bake for 15 mins until the tops are crisp and golden. Serve with salad and Tomato relish (see recipe, below).