Add the beef to a large bowl. Add the flour, 1 teaspoon salt, and ¼ teaspoon black pepper. Stir to coat the beef.

Melt the butter in a large skillet over medium-high heat. Add the beef and sear on both sides until golden brown. (No need to cook through, just getting the outside browned) Transfer the beef to the crock pot.

Add the celery, onion, carrots, and mushrooms to the same skillet. Sauté for about 5 minutes, until the vegetables begin to soften. This gives the vegetables a head start and ensures they cook through.

Transfer the vegetables to the crock pot.

Add the chicken broth, beef broth, tomato paste, pearl barley, bay leaves, onion powder, garlic powder, thyme, salt, and black pepper. Stir to combine.

Cover and cook on LOW for 7–8 hours, or until the beef is tender and the barley is cooked.

Remove the bay leaves. Stir and serve hot with crusty bread.