Return sauté pan to heat and add oil. Once oil is warmed, add mustard and cumin seeds and bring to a sizzle while stirring constantly. Once seeds start to pop, add curry leaves and fry for 30 seconds. Add onion and sauté until translucent and just starting to brown around edges, about 4 minutes. Stir in turmeric and let cook another minute before adding cooked quinoa and lemon zest. Cook, stirring constantly, to warm mixture, about 1 minute. Stir in lemon juice, cilantro, and peas. Remove mixture from heat and stir in toasted pistachios before transferring to a serving bowl.