Heat the oil in a medium saucepan over medium heat. Add the onions and cook until soft, taking care not to let them color, 3 to 5 minutes. Add the rice and stir to coat with the oil. Add 3 cups water and 1 teaspoon salt and bring to a boil. Reduce the heat to low, then cover and cook until all of the liquid has been absorbed, about 20 minutes. Remove from the heat and fluff with a fork to separate the grains. Let sit, covered, for 5 to 10 minutes.
Add the lemon juice and mint to the rice and toss to distribute evenly.
Serve hot, garnished with chopped mint.