How to Make It
In a large skillet, heat the 2 tablespoons of oil until shimmering. Add the broccoli rabe and cook over high heat, tossing, until just starting to wilt, about 2 minutes. Add the onion and cook, stirring, until softened, about 3 minutes longer. Stir in the fish sauce and vinegar and season with salt and pepper. Let cool slightly.
Light a grill or preheat a grill pan. Lightly brush both sides of the bread with oil, then grill over moderate heat until lightly charred on the bottom, about 1 minute. Flip the bread and top with the sliced cheese and broccoli rabe relish. Close the grill and cook until the cheese is melted and the bread is browned and crisp, about 2 minutes. Transfer to a platter, cut in half and serve right away.