Preheat the grill to high. Arrange the onion slices and garlic along the bottom of a medium stockpot. Place the bratwurst on top and then add the beer, water, coriander, caraway, mustard seeds, and ginger. Bring to a simmer over the grill grates or on a burner. Simmer the sausages in the mixture for about 10 minutes. Remove the pan from the heat and let the bratwurst sit in the liquid for 10 minutes.
Remove the sausages with a pair of tongs onto a platter. Remove the onions with a slotted spoon and place in a bowl. Grill the sausages until the casings are crisp and golden brown, about 3 to 4 minutes per side. Serve the bratwurst on the buns with the onions, mustard and sauerkraut, if desired. Garnish with pickles, pickled beets, and pickled eggs, if desired.
Red Cabbage Sauerkraut:
Preheat grill to high. In a medium saucepan, combine the oil, vinegar, water, and sugar over medium heat, and cook until the sugar has melted. Add the mustard seeds, garlic, and cabbage, and cook until the cabbage is soft, about 15 to 20 minutes. Season with salt and pepper, to taste.