In a small bowl, stir together the plain yogurt (about 1/2 cup for an average-sized serving) with the garlic and a pinch of salt. If you use more yogurt, a clove of garlic will still be enough, particularly if you have time to refrigerate it for a few hours or overnight, to allow the flavours to blend and intensify.
Smear the garlicky yogurt onto a platter. Arrange the cubed watermelon, olives, feta and tomatoes however you like, keeping them in a single layer so they’re easy to eat with a skewer or your fingers. Add a grinding of black pepper, and a drizzle of olive oil if you like. Scatter or garnish with whole or torn fresh mint or basil. Serve immediately.