How to Make It
Light a grill. Brush the onion slices on both sides with 2 tablespoons of the olive oil. Grill over low heat for about 20 minutes, turning only once or twice to keep the slices intact, until charred and tender. Brush the onion slices with the remaining 2 tablespoons olive oil during grilling. Let cool slightly. Cut the onions into 1/2 -inch dice and transfer to a large bowl.
In a small nonreactive saucepan, simmer the vinegar and brown sugar over low heat, stirring occasionally, until the sugar has dissolved, about 3 minutes. Pour the mixture over the onions and stir in the cayenne. Season with salt and black pepper. Serve the onion relish warm or at room temperature.