1. Peel onion and sweet potato and dice into 1/2-in. cubes. Heat oil in a large pan over medium heat. Add onion, potato, and salt and cook until onion begins to soften, about 2 minutes. Reduce heat to medium-low, cover, and cook, stirring occasionally, until potato is just tender, 8 to 9 minutes.
2. Increase heat to medium-high and add chorizo. Cook, breaking up sausage with the back of a spoon, until cooked through, 7 to 8 minutes.
3. Meanwhile, heat tortillas in an oven or a microwave, or char over an open flame until warmed and softened. Wrap in foil or a clean kitchen towel to keep warm.
4. Stack and thinly slice lettuce; combine in a bowl with juice of a lime half and cotija. Halve, peel, pit, and thinly slice avocados; put in another bowl and toss with juice from remaining lime half.
5. Spoon about 3 tbsp. chorizo-sweet potato mixture into each warm tortilla. Divide lettuce and avocado among tacos.
*Find Mexican chorizo and cotija at well-stocked grocery stores and Latino markets. Avoid the very soft, bright orange chorizo--it's too oily.