In a large Dutch oven, heat olive oil over medium-high heat. Add the beef cubes, seasoning with salt and pepper. Sear on all sides for 8 minutes. Remove from pot with a slotted spoon to a plate. Set aside.
Add the carrots, onions, mushrooms, garlic, salt and pepper to the remaining fat in the pot, scraping any brown bits from the bottom and cook over medium heat for 8-10 minutes, stirring often. Remove vegetables to a large bowl and set aside.
Add the beef cubes, tomatoes, beef broth, water, thyme sprigs and bay leaves to the pot. Bring to a boil over medium-high heat, uncovered. Once soup comes to a boil, lower heat to a simmer and cook until the beef is tender, about 1 1/2-2 hours. .
While the soup is simmering, bring three cups of water to a boil in a medium pot. Add barley. Cook, uncovered for 30 minutes. Drain excess liquid, if any, and set aside.
Discard the thyme sprigs and bay leaves from the pot and skim off fat. Add barley, reserved vegetables and thawed peas to the pot and cook for an additional 15 minutes. Season with salt and pepper. Pour into bowls and garnish with fresh parsley, if desired.