In large heavy saucepan, heat oil until shimmering. Add mustard seeds and cumin and cook for 30 seconds, until spices are fragrant and mustard seeds begin to pop. Stir in onion and gingerroot and sauté just until soft, about 2 minutes. Stir in remaining seasonings, cinnamon stick, and rice mixture until evenly coated with spices. Stir in water and bring to a gentle boil. Cover with lid and reduce heat to medium-low. Simmer for 40 to 45 minutes, or until peas are soft and tender.