In a small pan, over medium heat, add the vinegar, zest and juice of the lemon and a pinch of salt. Bring to a simmer and let the mixture reduce until it becomes syrupy, about 10 minutes.
Meanwhile, warm a grill pan over medium-high heat. Wash and trim the ends of the asparagus. Toss the asparagus lightly with olive oil, and season with salt and pepper, to taste. Grill asparagus 5 minutes, then flip and grill until slightly blackened on both sides, about 4 minutes more. Transfer the asparagus to a serving platter, drizzle with balsamic syrup and serve.