How to Make It
Prepare the Pickled Onions: Place onion slices in a small bowl or jar. Bring water, vinegar, sugar, and salt just to a boil in a small saucepan; pour over onion slices. Let stand at least 1 hour.
Prepare the Tacos: Cook chorizo in a large cast-iron skillet over medium-high, breaking up with a wooden spoon, until browned, 6 to 8 minutes. Add onion, garlic, and oregano and cook until softened, 4 to 5 minutes. Add zucchini, lime zest, lime juice, and salt. Cook, stirring occasionally, until zucchini is tender but not mushy, 3 to 4 minutes.
Divide mixture evenly among warm tortillas. Top with Pickled Onions.