Pork belly glaze and honey balsamic vinaigrette: combine the balsamic vinegar, honey, Dijon mustard, salt, and ground black pepper in a bowl. Mix to combine thoroughly.
Remove half of the balsamic mixture to a mason/ball jar or container with a lid. This is going to be the dressing and this part is a bit of a guesstimate but you want to look at the dark liquid in the jar and add that same amount of olive oil. It should be about 1/3 of a cup or so. Add the olive oil and shake the dressing to combine. There is salt and pepper in the dressing already, but you might want to taste it to make sure the dressing is seasoned properly.
Add the other half of the balsamic mixture to a small saucepan over medium heat. Bring the mixture to a boil then reduce to simmer. Cook, stirring occasionally, for 5 to 10 minutes or until the sauce starts to thicken. Once the mixture has reduced and thickened, remove it from the heat and add it to a small bowl or container with a lid to be used to glaze the pork while it is grilling.
Grilling pork and plums: preheat a charcoal or gas grill (or a stovetop grill pan) for 10 to 15 minutes.
Salt and pepper both sides of the pork belly slices and then add them to the hot grill. Cook the pork belly slices for 5 to 6 minutes on the first side.
Flip all of the pork belly slices and then paint glaze all over the cooked side of the pork. Cook the second side for an additional 4 minutes.
Add halved plums to the grill and cook for 2 minutes per side.
After you remove the plums, flip and glaze the pork belly. Continue flipping and glazing every minute until the pork reaches 140 F (60 C).
When the pork has hit the temperature, remove it from the grill. to a plate or bowl and if there is any glaze left pour it on top of the pork.
Sandwich assembly: toast the buns.
While the buns are toasting, chop the plums into bite-sized pieces. Then slice the brie into quarter-inch thick slices. Toss the arugula with a bit of honey balsamic dressing like you're making a simple salad.
To build the sandwich, add slices of pork on the bottom bun with brie on top of the pork.
Add a few bite-sized pieces of plum on the brie and then top the brie with dressed arugula.
Close the sandwich and serve.