Black pudding macaroni cheese toastie

Method

1. Heat 10g of the butter in a heavy-based saucepan over a medium heat, add the onions and a pinch of salt, then stir. Cover and cook gently for 15 minutes until completely soft. Turn up the heat, add the garlic and cook for another minute. Add the brown sugar and balsamic vinegar and stir to dissolve the sugar. Cook for another 4-5 minutes until caramelised. Set aside (or see tips).
2. Bring a large pan of salted water to the boil. Add the macaroni and cook according to the packet instructions (10-12 minutes). Drain well, then set aside.
3. Melt the remaining 15g butter in a saucepan and add the flour, stirring to make a paste. Cook for 1 minute, then add the mustard powder and cayenne pepper. Stir well, then remove from the heat. Pour in the milk, a little at a time, whisking between each addition to ensure there are no lumps. Put the pan back over the heat and bring to the boil, stirring constantly. Simmer for 2 minutes, stirring every now and then, until thickened. Remove from the heat, stir in the macaroni and 100g of the grated cheese until the cheese is melted, then set aside.
4. Heat a glug of oil in a frying pan and add the black pudding. Fry for 4-5 minutes until crisp, then transfer to a plate lined with kitchen paper to drain. Once drained, stir it into the macaroni cheese mixture to break it up a little and mix evenly .
5. Butter the sourdough slices well on both sides. Divide the macaroni cheese between 2 of the slices – spread to the edges in an even layer – and top with the rest of the grated cheese. Spread the other 2 slices of bread with the caramelised onion, then use to top the macaroni cheese sandwich (see Make Ahead).
6. Heat a large, heavy-based frying pan over a medium heat and add a splash of oil. Fry the sandwiches, one at a time, for 2-4 minutes on each side until golden, crisp and oozy. Weight each sandwich down with another frying pan while cooking to help them keep their shape and to ensure everything sticks together well. Transfer to a chopping board, cut in half and eat straightaway.