Heat oven to 220C/200C fan/gas 7. Wipe the mushrooms to remove any dirt, then turn upside down and remove the stalks. Place on a baking tray top-side down.
Fry the lardons for 8-10 mins until just starting to crisp up, then finely chop the shallot, mushroom stalks and garlic. Add to the bacon and fry on a moderate heat for about 5 mins until all the ingredients are cooked.
Take off the heat and add the breadcrumbs and sage, then stir to make an even mixture. Spoon into the hollows of the mushrooms and sprinkle with the grated cheese.
Bake for approximately 8-10 mins until the cheese has melted. Serve with crusty bread and a green salad.