Slow Cooker Classic Beef Stew

Ingredient:

• 2 tablespoons olive oil
• 2 pounds boneless chuck roast, trimmed and cut into 1 1/2-inch pieces
• 1 teaspoon black pepper
• 1 1/2 teaspoons kosher salt, divided
• 8 ounces cremini mushrooms, quartered
• 2 cups sliced red onion (from 1 red onion)
• 3 medium carrots, cut diagonally into 1/2-inch pieces
• 6 cloves garlic, smashed
• 1 cup dry red wine
• 1 bundle (tied with kitchen twine) thyme sprigs (about 4 large sprigs) and flat-leaf parsley sprigs (about 2 large sprigs)
• 2 bay leaves
• 12 ounces baby gold potatoes (cut any larger potatoes in half)
• 2 1/2 cups beef stock, divided
• 2 tablespoons all-purpose flour
• 10 ounces frozen pearl onions (about 1 3/4 cups), thawed
• 2 tablespoons whole-grain mustard
• 1 tablespoon red wine vinegar
• 2 tablespoons thinly sliced fresh chives