Heat the olive oil in a large pot over medium heat. Add the ground beef and cook until browned, breaking it up with a spoon as it cooks.
Add the diced onion, carrots, and sliced mushrooms to the pot. Cook for 5 minutes until the vegetables start to soften.
Stir in the minced garlic, tomato paste, and dried thyme. Cook for 1 minute until the tomato paste darkens slightly.
Pour in the beef broth and add the quick-cooking barley. Season with salt and pepper.
Bring the mixture to a boil, then reduce to a simmer. Cook uncovered for 15 to 18 minutes until the barley is tender and the stew has thickened.