Heat the olive oil in a large pot over medium heat. Add the onion and cook until it starts to soften.
Stir in the carrots, potatoes, and garlic. Cook for 3 minutes, stirring now and then.
Add the lentils, barley, vegetable broth, thyme, and bay leaves. Bring the mixture to a boil.
Reduce the heat to low, cover the pot, and simmer for 35 minutes until the lentils and barley are tender.
Remove the bay leaves. Taste the soup and season with salt and pepper as needed.
Ladle into bowls and top each serving with fresh parsley.