In a large Dutch oven, sauté red peppers, carrots, onions, celery and garlic in olive oil until tender. Stir in broth, rice, thyme, salt, pepper and cayenne pepper; bring to a boil. Reduce heat; cover and simmer for 20 to 25 minutes, or until vegetables and rice are tender. Cool for 30 minutes.
Puree in small batches in a food processor or blender, return to pan and add red pepper flakes. Heat through. Garnish with croutons.