Sear halloumi cheese in a griddle pan or electric grill over medium heat until golden brown, 3 to 4 minutes per side.
Combine tomato, onion, and watercress in a bowl and season with salt and pepper. Add vinegar and mint; toss well until vegetables are well covered. Align the halloumi cheese slices on top of the salad. Serve while cheese is still warm and gooey.