1. Heat a large skillet over medium-high heat. Sprinkle beef evenly with 1/4 teaspoon salt and 1/4 teaspoon pepper. Add 1 1/2 teaspoons oil to pan; swirl to coat. Add half of beef to pan; cook 6 minutes, turning until well browned on all sides. Remove beef from pan. Repeat procedure with 1 1/2 teaspoons oil and remaining beef; remove beef and any juices from pan.
2. Add remaining 1 tablespoon oil to pan; swirl to coat. Add onions and garlic; sauté 4 minutes. Add beer, scraping pan to loosen browned bits. Bring to a boil; cook 2 minutes. Stir in 1 cup stock, remaining 1 1/4 teaspoons salt, and remaining 3/4 teaspoon pepper. Bring to a simmer. Carefully pour mixture into a 6-quart electric slow cooker. Add beef, potatoes, carrots, thyme, and bay leaves. Cover and cook on LOW for 7 hours.
3. Combine remaining 1/4 cup stock and flour, stirring with a whisk. Stir flour mixture into stew; cook 15 minutes or until thickened. Stir in mustard and vinegar. Discard thyme sprigs and bay leaves. Sprinkle with parsley.