Preheat the grill for medium-low heat (300°F | 150°C) or set up a bed of dying coals under a campfire grate.
Pile the plum quarters on a large sheet of aluminum foil.
Top with butter, brown sugar, and nutmeg. Drizzle over the orange juice.
Fold up and seal the foil packet.
Place the plum foil packet over direct heat on the grill or your campfire grate. Grill until the plums are tender when pierced with a knife and just beginning to slump, 10 to 15 minutes.
Remove from the heat and open the top of the foil packet slightly to let steam escape. Let the plums sit in their juices for at least 10 minutes before serving. They will smell amazing and it will be incredibly difficult to be patient, but it's worth the wait.
Serve the warm plums with the syrup from the bottom of the pouch drizzled over the top.
Notes
Don't use overripe fruit--Extremely ripe fruit will break down too much during cooking. Choose plums that are ripe, but still firm.
How to check your plums for doneness--Gently open the foil packet from the top and give them a poke with a knife. If they're not ready, close up the foil packet and wait a few minutes. Don't try to increase the heat to rush things along as this will result in the plums breaking down.
Let the plums rest--This is the most important part of the whole recipe. The plums will be very hot, so they need to cool, and as they cool, the buttery juices will thicken up into a sauce.
Dietary--This recipe is suitable for gluten-free diets. To make it vegan, use plant-based butter.