Combine the bratwursts, onion, beer, garlic, seasoned salt, caraway seeds, paprika, and black pepper in a medium Dutch oven. Bring to a boil over medium-high heat. Reduce the heat to medium-low and simmer, uncovered, until the bratwursts are cooked through (a meat thermometer should register 165°F) and the onion is softened, about 15 minutes. Remove from the heat.
Preheat a grill to medium-high heat (about 450°F). Remove the bratwursts from the Dutch oven and transfer to the grill. Grill, uncovered, turning occasionally, until lightly charred, 4 to 6 minutes.
Meanwhile, spread the butter evenly along the cut (inside) edges of the hot dog buns. Grill the buns, cut side down, until lightly toasted, 1 to 2 minutes, if you like.
Place the bratwursts in the buns and top each with about ¼ cup cooked onions from the braising liquid. Serve with mustard.
