Place crust on large pizza pan or baking sheet. Spread olive oil evenly over pizza crust, add the grated Fontina cheese. Arrange leeks or onions atop the cheese. Add the chopped tomatoes and thyme. Season with salt to taste and add the pepper.
Bake pizza until crust is golden and cheese melts, about 15 minutes. Remove from the oven and cool in pan for 5 minutes. Cut into triangles or squares or diamonds or wedges and serve.