Rinse the rice with cold water until the water runs clear. Drain well in a fine-mesh sieve and set aside. This removes excess starch and helps the rice cook up fluffy and light.
Heat a medium saucepan with a tight-fitting lid, over medium heat. Add the oil or butter. When hot, add the rinsed rice and onion. Cook, stirring often, until the rice turns from translucent to a brighter white and the onion is soft, 3 to 4 minutes. Add the garlic and cook, stirring it around the pan until fragrant, but not browned, about 30 seconds.
Pour in the warm water or stock and stir, knocking any grains of rice that are stuck to the sides of the pan. Season with salt, reduce the heat to the lowest setting and cover with a lid. Cook, covered for 15 minutes.
Remove the pan from the heat and leave alone (still covered) for 10 minutes. Uncover and test a grain of rice. If the rice is still a little hard, place the lid back onto the rice and place over low heat for another 5 minutes. If the rice looks too dry, scatter a tablespoon or two of extra water over the rice before placing back onto the heat.
When the rice is done, scatter the lime juice and cilantro on top. Use a fork to gently fluff the rice and incorporate the lime juice and cilantro into the rice.