Using paper towels, blot beef dry and season generously season with salt and pepper.

In a large carbon steel, cast iron, or stainless steel skillet, heat oil over high heat until just smoking. Add half of the meat and cook until browned all over, flipping occasionally, 6 to 8 minutes.

Transfer seared meat and remaining raw meat to a slow cooker. Using a wooden spoon or flexible spatula, stir in onion, garlic, stock, porcini mushrooms, Worcestershire sauce, mustard, vinegar, sherry, bay leaves, ½ teaspoon Diamond Crystal kosher salt (for table salt, use half as much by volume) and ½ teaspoon freshly ground black pepper. Cover and cook on low until meat is fall-apart tender, 8 to 10 hours.

Once meat is cooked, use a slotted spoon to transfer to a bowl. Using your hands or 2 forks, pull meat into chunks, removing and discarding any fatty parts. Season to taste with salt and pepper; cover to keep warm.

In a small bowl, make a slurry by whisking together flour with 3 tablespoons of the remaining liquid from the slow cooker. Pour remaining liquid from slow cooker into a medium saucepan and bring to a simmer over medium-high heat. Whisk in slurry and simmer until sauce is thick enough to coat a spoon, 3 to 4 minutes. Whisk in sour cream until smooth, and stir in mushrooms, if using.

In a large saucepan of salted boiling water, cook pasta until just shy of al dente, 1 to 2 minutes less than package instructions. Using a colander, drain pasta and divide into individual serving bowls. Top with meat and spoon finished sauce on sauce. Garnish with parsley and serve.