Drain the sauerkraut and place it in a small disposable pan or grill-safe skillet. Spoon the yellow mustard, Dijon or whole-grain mustard, relish, pickles, and optional sauces into separate serving bowls.

Toss the sliced onions and bell peppers with the oil, kosher salt, black pepper, and optional brown sugar. Preheat the grill to medium-high heat and prepare a grill basket or perforated pan.

Grill the onions and peppers for 8 to 12 minutes, stirring frequently, until softened and lightly charred. Transfer them to a serving dish or keep them warm in a covered grill-safe pan.

Grill raw bratwurst and Italian sausages over medium heat, turning often, for about 12 to 18 minutes, or until browned and cooked through. Grill thinner kielbasa or smoked sausage for about 6 to 10 minutes, turning as needed.

Check that raw pork or beef sausage reaches 160°F internally, poultry sausage reaches 165°F, and fully cooked smoked sausage is heated to 165°F for serving. If desired, place cooked sausages in a pan with the beer and hold them over indirect heat without boiling.

Toast the cut sides of the rolls on the grill for 1 to 2 minutes. Watch them closely so they become lightly crisp without turning brittle.

Arrange the sausages, toasted rolls, grilled vegetables, sauerkraut, condiments, pickles, and optional herbs buffet-style. Let guests choose their sausage and build a sandwich with as many toppings as they like.