Preheat the slow cooker if necessary; see the manufacturer's instructions. Tip the macaroni into a bowl, cover with plenty of boiling water and leave to stand for 10 minutes while preparing the rest of the dish.
Heat the oil in a saucepan, add the onion and fry gently, stirring, for 5 minutes or until softened. Add the butter and, when melted, stir in the flour. Gradually mix in the milk and bring to the boil, stirring until smooth. Stir in the stock, 125 g (4 oz) of the cheese, nutmeg and salt and pepper and bring back to the boil, stirring.
Drain the macaroni and add to the slow cooker pot with the haddock and sweetcorn. Pour over the sauce and gently stir together. Cover with the lid and cook on low for 2–3 hours.
Stir the spinach into the macaroni, replace the lid and cook on low for 15 minutes. Lift the pot out of the housing using oven gloves and stir once more. Sprinkle the remaining cheese over the macaroni, then brown under a hot grill until the top is golden. Serve with grilled cherry tomatoes on the vine.