In a large skillet over medium heat, heat olive oil. Add onion, season with salt and pepper, and cook until slightly tender, 4 minutes. Push onions to edges of skillet and add brats to center of skillet. Prick each gently with a fork to release fat. Sear brats until golden, 2 minutes per side. Add beer and simmer until all the liquid has evaporated, 10 to 12 minutes.
Meanwhile, blanch cabbage: In a large pot of boiling water, use tongs to dip cabbage leaves in water for 30 seconds. Place on a paper towel–lined plate to dry. When cool enough to handle, cut a “V” at the base of the leaf to remove the toughest part of the stem.
Build wraps: Overlap 2 cabbage leaves, cut sides in, to create one large oval. Spread mustard in the center of cabbage leaves, then top with caramelized onions, sauerkraut, and brats. Fold the short ends of the cabbage leaves in, then tightly roll from one long end to the other.