Roast the garlic, serranos, and tomatoes over a flame or on a cast iron skillet, until blistered and slightly charred, turning sides as needed.
Place the vegetables in a food processor or blender. Remove the seeds from the serranos for a milder heat. Add onion, oregano, cumin, ancho chile, and ¼ cup of warm water. Blend until smooth.
In a large saucepan or a 12-inch Dutch oven, heat the corn oil over medium-high heat. Stir the rice in, and cook until golden brown and toasted, about 4 to 5 minutes, stirring constantly.
Stir in the serrano sauce, 1½ cups of water, and chicken bouillon. Bring to a boil, stirring constantly.
Cover, and turn the heat to the lowest setting. Simmer 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 for 10 minutes.
Fluff the rice with a fork. Season with salt and pepper, if desired. Serve warm.