Preheat the oven to 200C/400F/Gas 6. Grease a small ovenproof dish.
Whisk the egg in a bowl, then whisk in the milk.
Put the flour in a separate bowl, then slowly whisk in the egg mixture. Whisk in the mustard.
Put the sausges and tomatoes into the dish and pour over the batter.
Bake in the oven for 15-20 minutes, or until the batter is puffed up and golden-brown.
