Pat beef dry and season with salt and pepper. Heat oil in a large skillet over medium-high heat. Brown beef in batches, about 2-3 minutes per side, without overcrowding. Transfer browned beef to the slow cooker.
In the same skillet, add diced onion and cook until softened, about 3 minutes. Add sliced mushrooms and cook until they release their liquid, 4-5 minutes. Stir in minced garlic and cook 1 minute, then deglaze the pan with a splash of beef broth, scraping up browned bits.
Transfer the onion-mushroom mixture to the slow cooker. Add remaining beef broth, Worcestershire sauce, Dijon mustard, dried thyme, and bay leaf. Stir to combine. Cover and cook on low for 6-8 hours or on high for 4-5 hours, until beef is fork-tender and reaches an internal temperature of at least 145°F (63°C).
About 30 minutes before serving, remove and discard the bay leaf. In a bowl, whisk together room-temperature sour cream and flour. Ladle about 1/2 cup of hot liquid from the slow cooker into the sour cream mixture, whisking constantly to temper, then stir the mixture back into the slow cooker.
Stir the tempered sour cream into the stroganoff. Cook on high for 15-20 minutes until the sauce thickens slightly. If you prefer an even thicker sauce, whisk in the optional cornstarch slurry and cook another 5 minutes.
Cook egg noodles according to package directions, drain, and toss with a little butter. Spoon the stroganoff over the noodles, garnish with fresh parsley, and serve immediately.