Place onion, celery, and garlic cloves in a food processor, and pulse to chop.
Heat the oil in a Dutch oven over medium-high heat; add chopped vegetables. Saute 5 minutes; stir in clam juice and tomatoes. Bring to a boil; add thyme, pepper, and saffron. Reduce heat to medium; cook 10 minutes.
Add shrimp and mussels; cook 3 minutes or until shrimp are done and mussels open. Discard any unopened mussels. Remove from heat; stir in remaining ingredients.