Rinse quinoa until water runs clear. Drain and set aside.
In a medium size saucepan, heat vegetable oil. Fry onion, garlic, cumin seeds, and mustard seeds for about 2 minutes. Add quinoa and ground turmeric. Give it a good stir.
Add water and bring it to a boil. Add salt, cover, reduce heat and let quinoa cook for about 12 minutes.
Quickly add frozen green peas. If it appears too dry, add 1 to 2 tablespoons of water. Cover and continue to cook for another 3 to 4 minutes or until all water is absorbed. When done, quinoa appears soft and translucent with the germ ring visible along the outer edge of the grain. Turn off heat.
Add tomatoes and lemon juice. Stir to get everything well mixed.