Arrange the various melon on a large serving board.
Drain the halloumi and cut it into cubes. Pat the cubes dry with paper towels.
Heat the olive oil in a cast iron skillet over medium heat. Add the halloumi and cook for a few minutes without stirring, until the bottoms are well-browned. Turn the halloumi cubes with a pair of tongs, and brown them on the other sides.
Transfer the seared cheese on top of the melon. Drizzle with the vinaigrette. Serve immediately.