Heat the oil in a frying pan over medium heat. Add the sausage. Fry until browned on one side, about 5 minutes. Turn it over to fry on the other side and add the tomato, bacon and mushrooms.
When the tomato, bacon and mushrooms are just about cooked, crack the egg into the centre and allow to cook. You might want to add a little more oil just to crisp the edges. Toast the slice of bread while the egg cooks and then spread butter on it. Serve everything on a plate with the toast on the side.