Preheat panini pan, grill pan, or a heavy skillet. Spray with non stick cooking spray.
Lay one slice of bread on a flat work surface. Spread half of the mayonnaise on this slice of bread. Layer the spinach, cheese, ham, egg, bacon, and tomato. Spread remaining mayonnaise on the other slice of bread and add top to sandwich. Brush with half of the melted butter.
Lay sandwich, butter side down, on grill pan. Brush the remaining butter on top of sandwich.
Grill sandwich until bread is crisp, golden, and the cheese has melted. About 3 minutes on each side.
Transfer to a serving plate and cut in half.