Roast the garlic, serranos and tomatoes over a flame or on a cast iron skillet, until blistered and slightly charred, rotating as needed.
Place the vegetables in a food processor or blender. Remove the seeds from the serranos for a milder heat. Add the onion, oregano, cumin, ancho chile and ¼ cup warm water. Blend until smooth.
In a large saucepan or 12-inch Dutch oven, heat the corn oil over medium-high heat. Stir in the rice. Cook until the rice is golden brown and toasted, about 4 to 5 minutes, stirring constantly.
Stir in the serrano sauce, 1 ½ cups water and chicken bouillon. Bring to a boil, stirring constantly.
Cover, and turn the heat to the lowest setting. Cook for 20 minutes. Do not lift the lid!
Turn the heat off and remove the lid to release the steam. Quickly replace the lid and let rest 10 minutes.
Fluff the rice with a fork and salt and pepper, if desired. Serve warm.