In a large, heavy saucepan, heat the oil over medium-high heat. Add the onion and cook until browned, about 10 minutes. Add the garam masala and turmeric and stir to combine. Stir in 3 cups water and bring to a boil over high heat. Add the rice and salt and return to a boil, stirring briefly to break up any clumps. Lower the heat, cover the pan and simmer, without disturbing, until all the water is absorbed and the rice is cooked through, about 18 minutes.
Uncover the pan and mix in the peas while fluffing the rice with a fork; cover and set aside for 5 minutes, until the peas are heated through.