Tip macaroni into a bowl, cover with plenty of boiling water and leave to stand for 10 mins while you prepare the rest of the dish.

Heat oil in the saucepan, add the onion and fry gently, stirring, for 5 mins. Add the butter and when melted stir in the flour. Gradually mix in the milk and bring to the boil. Stir in the stock, 125g of the cheese and a little salt and pepper and bring to the boil stirring.

Drain the macaroni and add to the slow cooker pot with the haddock and sweetcorn. Pour over the sauce and stir together. Cover with the lid and cook on low for 2-3 hours

Stir in the spinach, replace the lid and cook on low for 15 mins. Lift the pot out and stir in the remaining cheese. Serve with tomatoes.