Preheat oven to 200°C (400°F) and line a baking sheet with parchment paper.
Group asparagus into bundles of three spears each.
Wrap one slice of bacon around each bundle and place them seam-side down on the baking sheet.
Brush bundles with melted butter and sprinkle with salt and pepper.
Bake for 20-25 minutes, until bacon is golden and crispy.
Remove from oven and brush lightly with balsamic vinegar before serving.