In a medium saucepan, bring the beer to simmer over medium-high heat. Reduce the heat to low and add the onions.

Preheat a grill to medium.

Over medium coals, grill the brat patties on both sides until cooked through. Add the brat patties to the beer and onions, and cook on low heat, just below simmer, (about 140 degrees F). Hold until ready to serve.

While the brats are grilling, add the sauerkraut to a saucepan over medium heat. Stir in the butter and water, and cook for about 30 minutes. Set aside. At assembly time, put a brat patty on each roll bottom and top with a few braised onions. Cover the onions with the top of the roll. Put a Swiss cheese triangle on top of each roll and secure with a frill pick. Arrange the rolls on serving plates.

Put a dollop of mustard on each plate and smear it with the back of a spoon. Fill 1-ounce shot glasses with sauerkraut and serve next to sliders on the plate. Serve with beer chasers.