Heat the oil in a large heavy-based frying pan or flameproof casserole dish set over a medium heat and fry the chorizo for 5 mins until crisp. Remove from the pan with a slotted spoon and set aside on a plate lined with kitchen paper.
Add the onion and fry for 8-10 mins, or until sticky and golden. Add the garlic, paprika, chipotle paste and ground coriander, then tip in the tomatoes, sugar and beans. Stir everything together and simmer, uncovered, for 15 mins or until reduced. Stir the chorizo back into the pan and season to taste.
Meanwhile, make the cheese sauce. Melt the butter in a pan until foaming, then stir in the flour and cook for 2 mins. Stir in the mustard. Remove from the heat and gradually whisk in the milk, a little at a time. Return the pan to the heat and cook for 3-4 mins more, whisking continuously until thick and smooth. Grate over some nutmeg, stir in 200g of the cheddar and season to taste.
Heat the oven to 200C/180C fan/ gas 6. Cook the pasta in a pan of boiling water for 2 mins less than the pack instructions. Drain and toss with the cheese sauce. Tip the cheesy pasta into a large baking dish, then drop the smoky bean mixture into it in spoonfuls. Top with the remaining cheddar and the mozzarella, then bake for 25 mins, or until golden and bubbling.
Meanwhile, make the guacamole. Put the avocado flesh in a bowl with the lime juice and zest, and roughly mash to a chunky texture. Stir through the chilli and half the coriander and season to taste. Serve the mac ‘n’ cheese with guacamole on the side, with the remaining coriander scattered over.