Begin by heating a generous splash of olive oil in a skillet over medium-high heat, or directly in the slow cooker if it has a browning function. Generously season your cubed boneless beef chuck roast with kosher salt and freshly ground black pepper. Sear the beef until it forms a rich, brown crust, about 4-5 minutes per side. Drain any excess fat before transferring the beef to the slow cooker.

In the slow cooker, layer in the sliced mushrooms, carrots, minced garlic, cubed potatoes, and chopped onion. Pour in low-sodium beef broth, dry red wine, tomato paste, and Dijon mustard. Stir everything gently to incorporate the ingredients, allowing the beef to soak up the hearty flavors.

Next, add pearl barley to the mixture along with a bay leaf and dried thyme for that cozy aroma. Give everything a good stir to ensure the barley and seasonings are evenly dispersed.

Cover the slow cooker and set it to cook on high for 4 hours. Then reduce the temperature to low and continue cooking for an additional 2 hours. The slow cooking process transforms the beef into tender morsels while allowing the flavors to meld beautifully.

Once the stew has finished cooking, remove the bay leaf and give it a gentle stir. Ladle the hot, hearty Slow Cooker Beef Barley Stew into bowls, garnishing each serving with chopped fresh parsley.