Mix together melted butter, Dijon, dried onion, and brown sugar; stir to combine.
(Tip: This step can be prepared in advance and stored in a sealed container. When ready to prepare sandwiches, warm over the campfire until butter is fully melted again.)
Prepare 8-12 sheets of heavy-duty foil (depending on how many sandwiches you are making). Spray each sheet of foil with cooking spray.
Place a Sandwich Roll or Slider Roll onto each sheet of foil. Divide the ham and/or turkey between the 8 (Sandwich) or 12 (Slider) Rolls. Top the deli meat with a slice of cheese, then close up the sandwich with the top of the roll.
(Tip: Mix and match the meat and cheese options to suit your preference. Here, we used ham with provolone (on Sandwich) and turkey with Swiss (on Slider).
Pour melted butter glaze evenly over the tops of the sandwiches (or brush on using a basting brush).
Wrap up each individual foil packet and fold the edges to seal. Place the packets on warm campfire coals. Cook, turning frequently, until the cheese is melted and the rolls are lightly toasted, 10-15 minutes.
(Tip: You may want to start checking every few minutes around the 10 minute mark, as the cooking time may vary depending on the strength of your fire.)