Prepare the grill for direct cooking over medium heat (180° to 230°C). If you're using a charcoal grill, keep a small part of the charcoal grate clear of briquettes as a safety zone of indirect heat.
Whisk the lager, 2 tablespoons mustard, the brown sugar, and coriander.
In a large disposable foil pan arrange the onions in an even layer. Add the bratwurst and pour the beer mixture over the top. Place the pan over direct medium heat, close the lid, and bring the mixture to a simmer. Cook until the bratwurst are evenly colored and no longer look raw, about 20 minutes. If the liquid starts to boil, temporarily move the pan over indirect heat to prevent the bratwurst from splitting open. Add the sauerkraut and jalapeños to the pan, and simmer for 2 to 3 minutes more.
Brush the cooking grates clean. Transfer the bratwurst from the pan to the cooking grates and grill over direct medium heat, with the lid closed, until browned and fully cooked (70ºC), 2 to 3 minutes, turning once. During the last 30 seconds to 1 minute of grilling time, toast the rolls, cut side down, over direct heat.
Place each brat in a roll. Using a slotted spoon, top with the sauerkraut mixture. Serve warm with more mustard on the side.
