Pat beef chunks dry with paper towels. Season with 1 teaspoon salt and the pepper. Heat olive oil in a large skillet over medium-high heat until shimmering. Working in batches, sear beef until deeply browned on all sides, about 8 minutes total per batch. Transfer to slow cooker.

Add sliced onions to the same skillet with a pinch of the remaining salt. Cook over medium heat, scraping up browned bits, until onions are soft and golden, about 12 minutes. They do not need to be fully caramelized - they will cook down in the slow cooker. Transfer to slow cooker.

Add beef broth, onion soup mix, Worcestershire, thyme, and bay leaf to the slow cooker. Stir to combine. Cover and cook on LOW for 8 hours or HIGH for 4 hours, until beef is fork-tender and shreds easily.

Remove bay leaf. Use two forks to shred beef directly in the slow cooker. It should fall apart with gentle pressure. Taste broth and adjust seasoning with remaining salt if needed.

Bring a large pot of salted water to a boil. Cook egg noodles according to package directions until just al dente, about 7 minutes. Drain well, reserving 1 cup pasta water.

Add hot noodles and butter to the slow cooker. Toss vigorously until butter melts and noodles are coated in the rich onion broth. If mixture seems dry, add reserved pasta water a splash at a time until loose and saucy.

Divide among shallow bowls. Top with shredded Gruyere so it melts from the heat, and finish with fresh parsley. Serve with crusty bread to soak up every drop.