Slow-Cooker Beefy Wild Mushroom and Barley Soup

1 1/2 lb boneless beef bottom round steak, trimmed of fat, cut into 1-inch cubes 5 cups sliced assorted fresh wild mushrooms, such as portabella, shiitake, oyster and/or porcini (about 10 oz total) 3 medium carrots, sliced (about 1 1/2 cups) 2 leeks, trimmed, cut in half lengthwise, rinsed and sliced crosswise (about 1 1/2 cups) 2 large cloves garlic, finely chopped 1/2 cup uncooked pearl barley 5 1/4 cups Progresso™ beef flavored broth (from two 32-oz cartons) 1/2 cup dry sherry or Progresso™ beef flavored broth 1/2 teaspoon dried thyme leaves 1/2 teaspoon salt 1/4 teaspoon pepper