Heat oil in a large pan over medium heat. Add mustard seeds and cook until they begin to pop. Add curry leaves, garam masala, chilli and onion, and cook until onion is golden and spices are aromatic. Add quinoa and peas, and stir to coat in spice mixture. Add stock and 1 cup (250ml) boiling water. Simmer over low heat for 25 minutes, then add chicken and spinach and heat through for 5 minutes. Season to taste and serve garnished with extra curry leaves.