Stir all dressing ingredients together. Set aside.
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 the outside of the 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.
Serve with Russian dressing on the side. Use about 1 tablespoon per sandwich.