HEAT a large frying pan over medium. Add oil, then garlic and onion. Cook until onion is soft, about 3 min. Add tomatoes, serrano, sugar and taco seasoning. Gently boil, stirring often, until most of the liquid has evaporated, 18 to 20 min. Reduce heat if mixture starts to splatter. Scrape into a blender. Purée until smooth.