Mix the mustard powder with a little lager in the bottom of a small pan to make a paste, then stir in the rest of the lager and add the butter. Heat gently to melt the butter, add the cheese and stir to melt, but do not let the mixture boil. Once smooth, add the chorizo.
Allow it to cool to room temperature then beat in the yolks. Place the monkfish on the tray and shape the rarebit to fit over.
Place in a pre-heated oven at 180°C for 10-15 minutes or until the fish is cooked. If necessary, grill the rarebit. Mix all the ingredients together for the dressing in a bowl. Place on top of the sliced tomatoes and cover with the dressing.