This is a perfect family autumn supper, made spooky for Halloween with the ghoulish potatoes on top! You can substitute the Guinness for beef stock if you prefer.
Make-ahead tip: This stew will keep in the fridge for three days and freezes perfectly for up to a month, so it's a great dish to cook ahead.
Preheat the oven to 325˚F.
Make the stew: Put the butter in a frying pan over medium heat. When the butter has melted, add the bacon, followed by the shallots. Cook until golden brown and transfer to a large baking dish. Add the beef to the frying pan, season with salt and pepper and cook until browned all over. Transfer to the baking dish. Add the mushrooms to the pan and cook for 2 minutes. Season to taste and transfer to the baking dish. Return the frying pan to the stove over medium heat and use a whisk to scrape off all the bits stuck to the bottom of the pan (this is where the flavor is). Pour in the Guinness and continue to whisk for another minute to deglaze the pan. Pour the Guinness and pan juices over the beef and vegetables in the baking dish. Add the bouquet garni, cover the baking dish and cook in the oven for 2 hours. Check the seasoning, remove the bouquet garni, and serve with roasted potatoes.
While the stew is cooking, make the ghoulish potatoes: Place the potatoes in a saucepan (put the largest ones at the bottom) and fill halfway with water. Cover the pan and place over a high heat. When the water begins to boil, drain off about half, leaving just enough for the potatoes to steam. When the potatoes are cooked, after about 30-40 minutes depending on the size, peel (hold them in a tea towel) and place in a warm bowl. Mash the potatoes, gradually adding the warm milk. Season with nutmeg, then add a knob of butter and season to taste with salt and pepper.
Place mashed potatoes in large re-sealable plastic bag; cut a 2-in. hole in one corner. Pipe ghost potatoes onto stew, and pop 2 peas at the top end of the whipped potatoes to look like eyes.