Heat olive oil in a large stock pot over medium-high heat. Cook onion, bell pepper, and garlic until lightly browned, about 5 minutes. Stir in tomatoes with their juice, vegetable broth, pepper, and red pepper flakes. Simmer, uncovered, for 15 minutes.
Add rice to soup and stir. Reduce heat, cover, and simmer 25 minutes, or until rice is tender.
When rice is cooked, whisk in peanut butter and return to a simmer, and serve. Garnish with chopped roasted peanuts, if desired.