Peel onion and sweet potato and dice into 1/2 inch 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 tender, 8 - 9 minutes.
Increase heat to medium-high and add chorizo. Cook, breaking up sausage with the back of a spoon until cooked through, 7 - 8 minutes.
Meanwhile, heat tortillas in an oven or microwave, or char over an open flame until warmed and softened. Wrap in foil or a clean kitchen towel to keep warm.
Stack and thinly slice lettuce; combine in a bowl with juice of a lime half and cotija. Halve, peel and pit and thinly slice avocados; put in another bowl and toss with juice from remaining lime juice.
Spoon about 3 tablespoons chorizo-sweet potato mixture into each warm tortilla. Divide lettuce and avocado among tacos.