Place a large pot, filled with water over high heat and bring to a boil. Add a generous amount of salt, and then add the asparagus and bring to a boil. Cook for 2-3 minutes, or until the asparagus are tender. For medium asparagus cooking time might be up to 5 minutes. Trasfer the asparagus to a large plate, lined with paper towel, pat dry and transfer to a serving platter. In a small mixing bowl combine the olive oil, vinegar, dijon mustard, sugar, garlic and lemon zest. Season with salt and pepper to taste and mix until well combined. Drizzle over the asparagus just before serving. Serve warm.