Place peanuts in a food processor; process until smooth (about 2 minutes), scraping sides of bowl once.
Heat the oil in a Dutch oven over medium-high heat. Add onion; sauté 7 minutes or until lightly browned. Add peanut butter, potato, and next 6 ingredients (potato through tomatoes); bring to a boil. Reduce heat; simmer, uncovered, 30 minutes or until potato is tender. Garnish with parsley, if desired.