First fry the bacon in a little butter until it's fairly crisp, then remove it to a warmed serving dish and chop it up quite small, and keep it warm.
Next fry the mushrooms in the bacon fat, tossing them around the pan and only cooking them very lightly. Then remove them and add them to the bacon. Now take a saucepan and melt about ½ oz (10 g) of butter in it. Whisk the eggs together in a basin, season them with salt and pepper, then stir them into the butter, using a wooden spoon - keep stirring over a medium heat and scrape the base (and especially the corners) of the pan to prevent the eggs sticking.
When they are half cooked (ie about half cooked egg and half liquid) remove the saucepan from the heat, add another knob of butter, then stir until the butter has melted and the eggs (which will go on cooking in the heat of the pan) are soft and glossy. Now mix in the mushroom and bacon, and pile the mixture on to toast or into a gratin dish.