In a large skillet over medium heat, melt the butter. Add the onions and peppers and sautee until soft, about 5 minutes. Stir in the brown sugar and El Diablo Jalapeno mustard. Stir in the sauerkraut. Remove from heat.
Make a foil pouch using two sheets of heavy duty foil and fold together the edges. Place the sauerkraut mix in the middle and pinch the edges closed, leaving room for steam.
Preheat a grill to medium-high. Prick holes in the brats.
Arrange the brats over a drip pan on the hot grill. Add the sauerkraut packet and grill, until brats are cooked through, about 20 minutes, flipping once, a little over half way through cook time.
Serve on toasted buns with kraut and extra El Diablo Jalapeno Mustard for those brave enough to try more heat.