Heat oven to 190C/170C fan/gas 5. Cook the gluten free macaroni following pack instructions, then drain in a colander.
Meanwhile, melt the lactose free butter in a large saucepan over a medium heat. Once melted, add the flour, mix well, then slowly add the lactose free milk, a splash at a time, mixing well after each addition to make a smooth sauce.
Once all the lactose free milk has been added, keep cooking and stirring until the sauce thickens. Stir in the mustard, lactose free cream cheese, cheddar and egg. Season well.
Tip the cooked gluten free macaroni into a deep baking dish (ours was 28 x 18cm) and pour over the cheese sauce, making sure the gluten free macaroni is well coated.
Bake for 20 mins until starting to turn lightly golden. Remove from the oven, leave to cool completely, then chill overnight.
The following day, heat oven to 190C/170C fan/gas 5 and slice the gluten free macaroni cheese into blocks. Transfer to 2 baking trays so the blocks have space around them.
Gently press a piece of mozzarella into the middle of each. Bake for 10-15 mins or until starting to crisp at the edges and the cheese has melted.
Put the blocks into the mini gluten free bread rolls with a few watercress sprigs, if you like. Serve immediately.