My son's girlfriend just made some this way. Mustard, mixed with shredded medium chedder, between sourdough. Darned good!
When I make them (when it's just me and the boys) I spread mayo inside and outside and slap 'em in a hot skillet. I only flip mine once. Patience Grasshopper.
You’ll say it’s gross until you try it. Don’t grill it with butter. Use mayo instead. Trust me.