Heat 2 tbsp of the oil in a large 30cm/12inch heavy-based frying pan and cook the quartered mushrooms over a high heat for 5 minutes, turning occasionally until golden and cooked through. Remove from the pan and keep warm. Wipe the pan clean and add a further 1 tbsp oil and cook the chipolata sausages over a moderate heat for 6-8 minutes, turning occasionally until golden and cooked through.
Add the tomatoes to the pan and cook for a further 1-2 minutes until the tomatoes have softened and some have lightly split.
Meanwhile beat the eggs with the parsley and mustard and season generously with salt and freshly ground black pepper. Return the mushrooms to the pan and stir the ingredients to evenly spread around the pan, then pour the eggs into the pan and cook gently over a medium heat for 3-4 minutes until the base is set. Meanwhile preheat the grill to hot and transfer the pan under the grill and cook for 4-5 minutes until golden and set. Cut into wedges to serve.