Cook asparagus in a pot of boiling salted water until crisp tender, about 5 minutes.
Drain in a colander.
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 about 5 minutes.To serve, drizzle the asparagus with the vinegar dressing and top with grated manchego cheese.