Preheat the oven to 200°C, fan 180°C, gas 6. Make the batter: put the flour in a bowl, add the eggs, then gradually beat in the milk until you have a smooth, thick batter, about the consistency of double cream. Stir in the mustard and set aside.
Pour the olive oil into a 30cm x 25cm x 5cm roasting tin and heat on the hob. Add the onions, red pepper and tomatoes and cook for 5 minutes to soften. Add the sausages for the final 2 minutes.
Pour the batter into the tin around the veg and sausages. Bake in the oven for 35-40 minutes until puffed up and golden. Cut into 4 and serve.