Meanwhile sauté tomatoes, onions, bell peppers and garlic in oil 5 minutes. Stir in stock, wine, saffron, salt, white pepper and bay leaves. Bring to a boil; reduce heat. Simmer, uncovered, 45 minutes.
Reduce heat to very low. Gradually whisk in butter just until melted and blended. Do not boil. Remove and discard bay leaves. Keep warm.