Cook asparagus in a pot of boiling salted water until crisp tender, about 5 minutes. Drain in a colander.
Mean while, heat oil in a large skillet over medium heat. Add the onions and garlic and cook, stirring until softened, about 3 minutes. Add the sherry vinegar, sugar, salt and pepper and stir for about 5 minutes.
To serve, drizzle the asparagus with the vinegar dressing and top with grated manchego cheese.