Grate the Gruyère on the large holes of a box grater directly over a large serving plate. Cut the tomato in half from top to bottom, remove the core, then cut crosswise into 1/2-inch-thick slices. Arrange the tomato half-moons in a circle around the mound of cheese.
Sprinkle the onion and parsley all over the cheese, then season everything with a pinch of salt and generous grindings of black pepper. Drizzle the oil, then the vinegar, all over the salad and serve immediately.
