Prepare the grill for direct cooking over medium heat (180°C 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 of mustard, the brown sugar and coriander.
In a large disposable grill-proof roasting pan arrange the onions in an even layer. Add the bratwurst and pour the beer mixture over the top. Place the roasting pan over direct medium heat, close the lid and bring the mixture to a simmer. Cook until the bratwurst are evenly coloured and no longer look raw, about 20 minutes. If the liquid starts to boil, temporarily move the roasting pan over indirect heat to prevent the bratwurst from splitting open. Add the sauerkraut and jalapenos to the roasting pan and simmer for 2 to 3 minutes more.
Brush the cooking grates clean. Transfer the bratwurst from the roasting 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.