I love recipes like this one.  Easy, not too many ingredients, and full of possible substitutions.  I love the Flat Iron steak for this but I’ve made it stew meat, chuck roast, sirloin steaks, and round steaks.  
It’s important not to use hulled, hull-less, quick-cooking, or pre-steamed barley in this recipe.  Just the cheap, basic barley.  Check the ingredient list just to make sure.  
The way this recipe builds on a trick to make a deep, flavorful base has become useful for me in so many other recipes.  When I don’t have time to brown the meat cooking the tomato paste with the onions and thyme and then deglazing the pan with wine creates a rich flavor base. 

Ingredients
2 tbs olive oil
2 onions, chopped
¼ cup tomato paste
1 tbs minced fresh thyme or 1 ts dried
½ cup dry red wine
1 (28 ounce) can diced tomatoes
4 cups beef broth
2 carrots peeled and chopped
1/3 cup soy sauce or bragg amino acid
1/4 cup pearl barley, rinsed
2 bay leaves
2 pounds flat iron  1 inch thick
Salt and pepper
2 tbs minced fresh parsley

Instructions

Heat oil in 12-inch skillet over medium heat until shimmering.  Add onions and cook until softened and lightly browned, 8 to 10 minutes.  Stir in tomato paste and thyme and cook until fragrant, about 30 seconds. Stir in wine, scraping up any browned bits; transfer to slow cooker.
Stir in tomatoes and their juice, broth, carrots, Braggs, barley, and bay leaves into the slow cooker.  Season steaks with salt and pepper and nestle into the slow cooker.  Cover and cook until the meat is tender, 9-10 hours on low or 6-7 hours on high.
Discard the bay leaves.  Transfer the steaks to a cutting board, let cool slightly, then shred into bite-sized pieces using 2 forks.  Stir beef into the soup and let sit until heated through, about 5 minutes.  Stir in parsley and season with salt and pepper to taste.  Serve. 
America’s Test Kitchen – The Complete Slow Cooker