1Heat vegetable oil in a large skillet over medium-high heat. Pat beef dry with paper towels, season generously with salt and pepper, then toss with the flour until lightly coated.
2Working in batches to avoid crowding, brown the beef cubes on all sides, about 2 minutes per side. Transfer to slow cooker.
3In the same skillet, cook sliced onions over medium heat, stirring often, until golden and softened, about 8 minutes. Add garlic and cook 1 minute more. Stir in tomato paste and cook 1 minute.
4Transfer onion mixture to slow cooker. Add beef broth, worcestershire sauce, mushrooms, thyme, and bay leaves. Stir to combine.
5Cover and cook on low for 7 to 8 hours, or on high for 4 to 5 hours, until beef is tender.
6In a small bowl, whisk together sour cream and dijon mustard. Turn slow cooker to high if on low. Stir in sour cream mixture. Cook uncovered for 15 minutes until slightly thickened.
7Discard thyme sprigs and bay leaves. Taste and adjust seasoning with salt and pepper. Serve over egg noodles, mashed potatoes, or with crusty bread.