Heat 1 tbsp. oil in a large skillet over medium-high. Sprinkle beef with pepper and 1 tsp salt. Add half of beef to skillet, and cook, until browned on 2 sides, 3 to 4 minutes per side. Transfer browned beef to a 6-quart slow cooker. Repeat with remaining 1 tbsp. oil and remaining half of beef.

Add mushrooms to skillet, and cook, stirring often, until browned, 5 minutes. Transfer to a plate, cover, and refrigerate.

Add red onion and carrots to skillet. Cook, stirring occasionally, until red onion begins to brown, about 5 minutes. Add garlic; cook, stirring often, until fragrant, about 1 minute.

Add red wine; cook until liquid has almost evaporated, about 4 minutes, stirring and scraping to loosen browned bits from bottom of skillet. Add red wine mixture to slow cooker with beef.

Stir in herb bundle, bay leaves, potatoes, and 2 cups stock. Cover and cook on LOW for 7 hours.

Whisk together flour and remaining ½ c. stock in a small bowl; whisk into mixture in slow cooker until incorporated.

Stir mushrooms, pearl onions, mustard, and red wine vinegar into beef mixture. Increase slow cooker heat to HIGH, and cook, uncovered, until slightly thickened, about 15 minutes.

Turn off slow cooker; remove and discard herb bundle and bay leaves. Ladle stew into bowls, and sprinkle with chives.