Rinse the rice in a fine mesh strainer until the water runs clear. Set aside.
In a large saucepan over medium-high heat, add the oil. Once hot, add the rice and stir to combine. Cook over medium heat, stirring frequently, until the rice is lightly golden brown all over (about 10 minutes).
Add tomato sauce, garlic, and diced onion to the pan. Stir. Add tomato bullion, salt, carrots, peas, and water (and serrano peppers, if using. They add a little flavor, but not spice.).
Bring to a boil, then cover, reduce heat to low and cook for about 20 minutes or until the water is completely absorbed. Remove from heat and allow to rest for 5 minutes before fluffing with a fork.