Prepare the grill for direct cooking over medium heat (350° to 450°F).
Whisk the lager, 2 tablespoons mustard, 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. Simmer for about 20 minutes, turning the brats two or three times. Add the sauerkraut and jalapeños to the pan, and simmer for 2 to 3 minutes more.
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 (160°F), 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 and top with mustard and the sauerkraut mixture. Serve warm.