Combine all the ingredients in a high powered blender and blend for 2 minutes until smooth. Adjust salt and pepper and set aside.
For the Salad
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.
Notes
Can't find halloumi? Sometimes stores sell it under the name "bread cheese."