Spread amount of mustard desired on each slice of bread. On one slice, arrange the remaining ingredients over mustard. Top with other slice, mustard-side down.
Butter outsides of sandwich. Cook in a preheated panini grill. Or, cook in a skillet over medium-low heat until golden on one side. Flip and cook until second side is lightly browned and cheese is well melted. Also tasty with Russian sauce.