Arrange grapes single layer on parchment-lined baking sheet. Roast grapes 45-55 minutes, or until grapes begin to split. Remove from oven and transfer to blender. Remove cap from blender lid and cover with a clean kitchen towel or otherwise vent blender to let steam release. Blend until smooth, then strain through a metal strainer to remove skins.
Transfer the juice to a pot or pan and add honey and lemon juice. Bring to a simmer on the stove and simmer until combined.
Store in airtight container in refrigerator for up to 2 weeks or 3 months in freezer.