Heat the olive oil in a large pot over medium-high heat. Add the lamb cubes and brown them on all sides for about 6 minutes. Remove the lamb and set it aside.
Lower the heat to medium. Add the diced onion and minced garlic to the pot. Cook for 3 minutes until the onion softens.
Return the lamb to the pot. Stir in the sliced carrots, rinsed barley, beef broth, thyme leaves, salt, and pepper.
Bring the mixture to a boil, then reduce to a low simmer. Cover the pot and cook for 60 minutes, stirring occasionally, until the barley is tender and the lamb is soft.
Taste and adjust salt and pepper if needed before serving.