How to Make It
When the embers in your grill are beginning to die down, carefully tuck in the onions at the edge. Cook until charred and tender throughout, at least 3 hours, depending on the volume and heat of your embers. Let cool completely, then scoop the tender, smoky flesh out of the charred skins (you should have about 1/2 cup). Finely chop the onion flesh.
In a medium bowl, whisk the smoked onion with the vinegar, mustard and sugar. While whisking constantly, slowly drizzle in the oil until emulsified. Season with salt and pepper.