Heat oil in a large saucepan or medium soup pot over medium heat. Stir in onion and celery; cook until the onion is softened and translucent, 8 to 12 minutes; do not brown.
Stir in curry powder, then add carrots, apples and bay leaf. Stir well over medium heat for 2 minutes, then add broth and salt. Bring the mixture to a low boil, then reduce the heat to low. Cover tightly and simmer until the carrots and apples are tender, 20 to 25 minutes.
Remove the bay leaf. Using a large slotted spoon, transfer the soup solids to a food processor, adding about 1/2 cup of the broth; process to a smooth puree. Pour the puree back into the soup. Reheat and season with pepper. Serve piping hot, garnishing each serving, if you like, with a dab of yogurt and a sprinkle of fresh herbs.