1 Put the cheese and milk in a heavy based pan. Bring to a simmer but do not boil, or the mixture will split. Remove from the heat. Stir in the flour, mustard and breadcrumbs, return to the heat and mix until the mixture leaves the sides of the pan. Allow to cool slightly.
2 Add the beer and Worcestershire sauce. Season to taste with salt and pepper. Beat in the egg yolks and leave to cool and thicken. Cover with cling film.
3 Preheat the oven to 200°C (180° fan) 400°F gas 6. Butter a baking dish.
4 Place the tomatoes in the dish. Place the fish on top and season with salt and pepper. Slice the rarebit into 4 and place on top of the fish. Cook for 10-15 minutes until the fish is cooked and the rarebit is bubbling.