Preheat the oven to 500°F, or as high as your oven will go. Place the pizza dough on a lightly floured work surface and roll out into a thin 15-inch circle. Transfer to a large baking sheet and drizzle the olive oil over the top. Sprinkle the cheese and red onion slices evenly over the surface of the dough and season with salt and pepper. Bake for 12 to 15 minutes, until the crust is golden brown and the cheese is melted. Remove from the oven and scatter the tomatoes, cucumber, olives, and feta cheese over the crust. Drizzle the tzatziki sauce over the top and serve.