Lay out eight slices of bread. Spread mayonnaise on one side of four slices and honey Dijon mustard on the other four.

Layer the mayonnaise slices with deli turkey, red onion, and Monterey Jack cheese. Season with salt and pepper before topping with the mustard slices.

Spread butter on the outside of each sandwich.

Preheat skillet over medium heat, place sandwiches in the skillet, and cook for 3-4 minutes on each side.

Remove from skillet, allow to cool for a minute, then slice diagonally and serve warm.