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Heat oven to 200C/180C fan/gas 6. Mix 3 tbsp of the milk with the cornflour and warm the remaining milk in a pan. Meanwhile, bring a pan of water to the boil, tip in the macaroni and cook for 6 mins, stirring occasionally.
To make the sauce, stir the cornflour mix into the warm milk and keep stirring over a low heat until thick and smooth. Remove from the heat and stir in most of the cheddar (leaving a little to sprinkle over at the end) along with the Grana Padano and pepper to taste.
Drain the macaroni well, then stir into the sauce. Squeeze the excess moisture from the spinach by pressing it into a fine sieve. Divide the spinach between four individual ovenproof dishes, followed by the tomatoes. Spoon the macaroni cheese over the veg, then finish with a sprinkle of the remaining cheddar. Bake for 15-20 mins or until the sauce bubbles and browns on top.