Heat the oil a large saucepan over medium heat. Add the onion and garlic and saute until tender and translucent. Add the rice, chicken broth and the sazon packet and stir until the rice is coated with the oil and toasted. Stir in the tomatoes and season with salt and pepper. Reduce the heat to low and cover with a lid. Cook until the rice is tender and all of the liquid is absorbed, about 15 to 20 minutes. Remove the lid and fluff with fork. Gently fold in cilantro and transfer to a serving bowl.