How to Make It
Heat oil in a large Dutch oven over medium-high. Sprinkle beef with pepper and 1 teaspoon salt. Add half of beef to Dutch oven, and cook until well browned on 2 sides, 3 to 4 minutes per side. Transfer browned beef to a plate. Repeat with remaining beef. Set beef aside.
dd mushrooms to Dutch oven, and cook, stirring often, until browned, about 5 minutes. Transfer to a plate. Add red onion and carrots to Dutch oven. Cook, stirring occasionally, until 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, 6 to 7 minutes, stirring and scraping to loosen browned bits from bottom of Dutch oven.
Whisk together flour and 1/2 cup of the stock in a small bowl; whisk into mixture in Dutch oven until incorporated. Stir in beef, herb bundle, bay leaves, remaining 2 cups stock, and remaining 1/2 teaspoon salt; bring to a boil. Reduce heat to medium-low, cover, and simmer until beef is tender when pierced with a fork, about 50 minutes. Add potatoes; cover and cook until potatoes are tender, about 20 minutes.
Stir mushrooms, pearl onions, mustard, and red wine vinegar into beef mixture. Cook, uncovered, until pearl onions are tender, 8 to 10 minutes. Remove from heat; remove and discard herb bundle and bay leaves. Ladle stew into bowls, and sprinkle with chives.