Heat a large skillet over medium-high heat. 1/4 teaspoon salt and 1/4 teaspoon pepper, evenly sprinkled over the beef. Swirl 1 1/2 teaspoon's oil into the pan to coat.
Cook for 6 minutes, turning once until the beef is well browned on all sides. Remove the beef from the pan.
Repeat with the remaining 1 1/2 teaspoons oil and beef; remove beef and any juices from the pan.
Swirl in the remaining 1 tablespoon oil to coat. Sauté the onions and garlic for 4 minutes.
Scrape the pan to loosen browned bits before adding beer. Bring to a boil and cook for 2 minutes. Add 1 cup stock, the remaining 1 1/4 teaspoons salt, and the remaining 3/4 teaspoon pepper. Bring to a boil.
Pour the mixture into a 6-quart electric slow cooker with care. Combine the beef, potatoes, carrots, thyme, and bay leaves in a large mixing bowl.
Cook on LOW for 7 hours, covered.
Whisk together the remaining 1/4 cup stock and the flour. Stir the flour mixture into the stew and cook for 15 minutes, or until thickened.
Mix in the mustard and vinegar. Remove the thyme sprigs and bay leaves. Garnish with parsley.