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Take the lobster and twist the claws off, then use a rolling pin to crack the claws and pick the meat out. Peel the tail, exposing and removing the fleshy meat. Roughly chop all of the meat, then cover and set aside in a bowl in the fridge. Bash all the pieces of shell with the rolling pin to break them up a bit.
Heat the oil in large frying pan over a high heat, add the onion and a large pinch of salt and cook for around five minutes until softened and well coloured, then add the lobster shell and fry for a couple of minutes more. Add the cayenne pepper and tomato purée and stir to coat everything, the turn the heat down to medium and add the milk. Stir well, then remove from the heat when it is just coming to a simmer and set aside for 30 mins to infuse. Meanwhile, mix the garlic, olive oil and lemon zest together with some seasoning to make the breadcrumb topping, then set aside. Strain the milk mixture into a jug and discard the shell.
Heat oven to 180C/160C fan/gas 4. Boil the macaroni in salted water until it has a slight bite (around two minutes less than on the packet instructions), drain and set aside. Melt the butter in a large frying pan over a medium heat until foaming, then add the wine and cook for 5 minutes until the wine has reduced by half, then whisk in the flour to make a sandy paste. Turn down the heat and pour the strained milk into the pan a little at a time, whisking constantly and allowing each addition of milk to be fully absorbed before adding any more.
Add the chopped lobster meat, mustard, crab and cheese. Stir well until the cheese has melted, then season to taste. Tip in the macaroni and stir to coat everything in the sauce. Tip into an ovenproof baking dish (or six small ones) and top with a sprinkling of grated cheese and the garlic breadcrumbs. Bake for 20 mins until golden and bubbling, then allow to cool a little before serving.