Preheat your oven to 170℃
To make the sauce, melt the butter in a saucepan, then add milk to warm through. Before the milk starts boiling, mix the cornflour in a little bit more cold milk (about 50ml) and add to the pan, stirring until it thickens. Add the Worcestershire sauce, mustard and grated cheese and season to taste. For for 2 minutes and remove from the heat. You could also make white sauce with wheat flour using your usual method if you are not worried about gluten.
In a large non-stick frying pan, fry the garlic in olive oil over medium heat until JUST coloured (watch it carefully - it's goes quickly!!). Add the tomatoes and toss in the mixture. Cook until they start to soften. Tip the mixture into a roasting tray and spread out.
In the same pan, add some more olive oil and then cook the spinach until wilted (drain off any excess liquid). Season with salt, pepper and nutmeg and spread it as evenly as you can over the tomato layer.
Top with portioned haddock.
Pour the warm rarebit sauce evenly over the fish (smear with the back of a spoon if necessary), then sprinkle with grated cheese.
Bake in the oven for about 30 minutes or until it's all golden and bubbly. Make sure the fish is cooked through (it should flake easily).
Serve piping hot with mashed potatoes.