Pat beef dry with paper towels and sprinkle with salt and pepper. In a large skillet over medium-high heat, heat oil until shimmering.
Add the beef and let cook, without stirring, until brown on one side, about 2 to 3 minutes. Stir beef and continue cooking until brown all over, 1 to 2 minutes longer. Transfer beef to the slow cooker, leaving the drippings in the skillet.
Add onion, carrots, celery, and mushrooms to the saucepan and sauté until the vegetables have softened and the mushrooms have released most of their liquid, about 5 to 7 minutes. Stir in the garlic and thyme until fragrant, about 30 seconds. Add to the slow cooker.
Add beef broth, chicken broth, water, barley, bay leaf, 1 teaspoon salt, and ½ teaspoon pepper to the slow cooker. Cover and cook on HIGH for 3 to 4 hours or LOW for 7 to 8 hours. Remove bay leaf and season to taste with salt and pepper.
Boneless beef chuck or stew meat: Chuck roast and stew meat are not only inexpensive, but also have a lot of fat marbled within the meat; an asset here. The longer cook time called for in the slow cooker recipe allows the fat to break down into meltingly tender chunks of beef and a fat that melts into the rich, full-bodied broth. Names can vary, so look for any of these beef cuts: boneless beef chuck roast, chuck pot roast, chuck shoulder, bottom eye, round roast, rump roast, top round, round tip roast, English roast, or pot roast.
Mushrooms: Cremini mushrooms, white button mushrooms, or baby bellas; choose any one or a mix of all. If they're already small, no need to slice the 'shrooms in half.
Beef broth and chicken broth: I use refrigerated beef base to make broth. Similar to bullion cubes, beef base is highly concentrated beef stock. You can find it in powder or paste forms. I love “Better than Bouillon” brand, a paste which is widely available and tastes delicious. For the chicken broth, buy your favorite store-bought brand in boxes or cans, or make your own chicken broth if you like.
Pearled barley: Also known as pearl barley, this grain is whole barley minus its fibrous outer hull. Pearl barley gets its shiny name because after the hull is removed, the grains are polished to remove part or all of the bran layer (making it quicker and easier to cook).
Yield: This Slow Cooker Beef and Barley Soup makes 8 entree-sized servings. Paired with a salad and warm bread or a half sandwich, this makes for a satisfying and soul-feeding meal.
Storage: Store leftovers covered in the refrigerator for up to 4 days.
Freezer: Cool the soup completely, then pack into freezer-safe containers (I like mason jars; leave 1/2-inch headspace for expansion). Label, date, and freeze for up to 2 months. Thaw overnight in the refrigerator.