In a large pot or Dutch oven, heat the oil over medium heat.
When the oil is hot, add the carrot, celery, onion, garlic, salt, black pepper, paprika, and cayenne (if using). Cook, stirring, until the vegetables are tender, 5 to 7 minutes.
Increase the heat to medium-high and add the broth, chicken, lemon juice, and bay leaf. Bring to a boil, then reduce the heat to a light simmer.
Add the couscous, lemon zest, parsley, and oregano. Cook, uncovered and stirring occasionally, until the couscous is tender, about 10 minutes.
Stir in the dill. Taste and add more salt, if desired. Discard the bay leaf.
Ladle the soup into bowls to serve and garnish with more dill.