Prepare the Filling (can be done up to 3 days in advance):

1. Heat 2 tablespoons of olive oil in a large pot over medium heat. Working in two batches to avoid crowding the pan, sear the beef until well-browned on all sides. Use a slotted spoon to transfer the seared beef to a bowl, and set aside.
2. Next, add the remaining tablespoon of oil, and fry the bacon until it starts to brown. Add the onion and carrot, and saute until the onion is soft and golden, about 8-10 minutes. Add the garlic, and cook until fragrant, about 1 minute longer.
3. Stir in the stout, using a wooden spoon to scrape up any browned bits that are clinging to the bottom of the pan, then add the beef stock, tomato paste, dried thyme and bay leaf. Stir in the mushrooms, along with the reserved beef and any juices that may have accumulated in the bowl. (Don’t worry if there isn’t quite enough liquid to cover everything in the pan - this is totally the way it’s supposed to be.)
4. Bring the mixture to a simmer, then reduce heat to low and cook, covered, for 1 hour 45 minutes or until the beef is tender.
5. In a small bowl, whisk together the flour and water until smooth. Stir into the stew, and cook for 15-20 minutes longer, or until the sauce is thick and rich. Stir in the frozen peas, and simmer for another minute or two to warm them through.
6. Remove from heat, and discard the bay leaf. Taste and adjust the seasoning, if necessary. Set aside to cool while you prepare the mashed potatoes. (You can also prepare the filling in advance and store in the refrigerator for up to 3 days, then bring up to room temperature before assembling the pies.)

Make the Topping and Assemble the Pies:

1. Preheat oven to 375F.
2. In a large pot of boiling salted water, cook the potatoes and garlic until the potatoes pierce easily with a fork, about 15 minutes. Drain, and stir in the cream and butter. Mash until smooth, then stir in the cheese, mixing until it melts and incorporates into the potatoes. Season to taste with salt and pepper.
3. Divide the beef stew between four oven-safe bowls (or into a casserole dish, if you prefer to make one big pie). Spoon the mashed potatoes onto the filling, smoothing out into an even layer. Bake in preheated oven for 20-25 minutes, or until the filling is bubbling hot and the mashed potatoes are golden-brown. Let stand for 5 minutes to cool off slightly, then serve.