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Tip the pasta, milk and 250ml water into a large saucepan and bring to the boil. Reduce the heat to a simmer, then cover and cook for 2 mins. Remove from the heat and leave to stand for 10-12 mins, or until the pasta has absorbed most of the liquid. Add the frozen spinach, cover again and leave to stand for 5 mins until the spinach has defrosted, then stir to combine. Stir in all the cheese and season to taste.
Meanwhile, tip the crumbled sausage into a cold frying pan, then set over a medium heat. When the pan is hot and the crumb has started to sizzle, stir and add the rosemary, if using. When the sausage is crisp, spoon half of it over the pasta and stir to combine, leaving any excess oil behind in the pan. Fry the bread cubes in the remaining sausage and oil until crisp.
Reheat the mac & cheese gently if needed, then divide between two bowls and scatter with the bread and sausage crumb before serving.