In a small bowl, combine the garlic, curry powder, onion powder, turmeric, thyme and pepper; set aside. In a large pot over medium heat, add the oil. Add the leeks and cook, stirring occasionally until soft, about 5 minutes. Add the carrots and celery and cook, stirring occasionally, for 5 minutes. Add the garlic mixture and stir for one minute. Add the tomato paste and cook, stirring continuously, for 2 minutes. Stir in the broth and bay leaves. Bring to a boil.
Stir in the peas and lentils and bring back to a boil. Reduce the heat, partially cover the pot, and simmer, stirring occasionally and scraping the bottom of the pot to prevent scorching, until the peas are soft, about one hour. Add the sweet potato and salt (and a bit of water or broth if the soup is thicker than desired) and cook, stirring occasionally, until the sweet potato is soft, about 20 minutes. Serve with a dollop of yogurt and sprinkling of green onions, if desired.