Place the rice in a bowl with warm water and set aside for 15 minutes.
Meanwhile, place the chopped onion, clove of garlic and ½ cup of cold water in a blender and process until you have a smooth mixture.
After 15 minutes, drain the water and rinse the rice with the help of a colander and cold water until water is clear. Make sure to drain well and shake any excess water, it has to be as dry as possible.
Add vegetable oil to a saucepan or Cazuela and place over a medium-high heat. Heat oil and add the rice. Fry the rice, stirring constantly until it becomes transparent, about 12 minutes. With the help of a spoon, drain any excess oil after this step.
Add the Onion and garlic mixture to the rice and stir (be careful not to break the rice grains).
Add the hot water, parsley sprigs, and Serrano pepper (if using) and a few drops of lime juice.
When the water starts boiling, lower the heat to the lowest temperature, season with salt to your own taste and cover the saucepan. Do not remove the lid.
After 20 minutes the water should be absorbed almost completely. The rice should look cooked and soft. Remove from heat and let stand for about 10 minutes. Discard the sprig of parsley and serve. Rice will keep cooking with its own heat.