Place a slice of cheese on one slice of the bread. Arrange onions and tomatoes. Top with another slice of cheese. Place another bread slice.
Drizzle some oil or butter on panini maker. Place the sandwich and then drizzle some more oil or butter on the sandwich. Grill to perfection.
You can also make this in a pan. Drizzle some oil and butter in a pan. Place the sandwich and hold it down with a flat spoon. Flip and repeat the same with the other side until the cheese is melted and the crust is crisp.