Special equipment: a spiral slicer
For the bratwurst: Prepare a gas grill for medium-high heat. Grill the bratwurst until cooked through (160 degrees F on an instant-read thermometer), turning once, 20 to 25 minutes.
For the slaw: Meanwhile, in a large bowl, whisk together the vinegar, honey, mustard, dill, 1/2 teaspoon salt and a generous grinding of black pepper. Whisk in the olive oil to make a dressing.
With a spiral slicer, cut the apples and carrot into spirals; add to the dressing. Add the scallions and toss well. Refrigerate the slaw until ready to serve.
When the brats are nearly finished cooking, brush the cut sides of the pretzel buns lightly with melted butter and grill, cut-side down, on the upper shelf until crisp at the edges, 1 to 2 minutes. Turn the buns cut-side up and top with the sliced brie; grill until the brie just begins to melt, about 1 minute.
Serve the bratwurst on the buns, topped with slaw. Serve any extra slaw on the side.