Heat oil in a saucepan over medium heat. Cook and stir rice and onion in the hot oil until browned, about 5 minutes; season with salt, cumin, and pepper. Pour water over the rice mixture. Stir tomato sauce and chicken bouillon into the water. Increase heat to medium-high, place a cover on the saucepan, and bring to a boil. Add serrano chile pepper and continue cooking at a boil for 10 minutes. Reduce heat to medium-low until the rice is tender and the water is absorbed, 15 to 20 minutes more.