1. Cook bacon with 2 tbsp oil in a large pot over medium heat until crisp and browned. Drain on paper towels. Pour fat into a bowl, and reserve.
2. Wipe out pot with paper towels. Add a tablespoon reserved fat to pot, and heat over medium-high heat until hot.
3. Season beef with 2 tsp salt and 1 tsp pepper. Working in batches, cook beef until browned on all sides, 2-3 minutes. Using a slotted spoon, transfer beef to a bowl. Add more reserved fat between batches if needed.
4. Pour fat from pot into the bowl, and reserve. Add 1 cup stock to pot, and cook, stirring and scraping bottom, for 1 minute. Pour over meat.
5. Heat reserved fat in pot. Add yellow onion and garlic, and cook for 3 minutes. Stir in mushrooms, and cook for 2 minutes. If bottom of pan begins to burn or onions begin to stick, stir in about 1/4 cup of the remaining stock. Sprinkle flour over vegetables, add mustard, and cook, stirring constantly, for 1 minute.
6. Return beef to pot. Add remaining stock, the stout, thyme, and bay leaves. Bring to a boil. Reduce heat, and simmer, partially covered, for 1 hour 20 minutes.
7. Add potatoes and cipollini or pearl onions, and partially cover. Simmer until potatoes and onions are tender but not mushy, about 25 minutes. Stir in bacon. Season with salt and pepper.
8. Bring a large pot of water to a boil. Add 1 tbsp salt. Cook noodles according to package instructions. Drain. Add butter and dill to warm noodles, and toss. Season with salt and pepper.
9. Combine horseradish and vinegar. Ladle stew over noodles, and garnish with carrots, dill, and horseradish.
