In a small saucepan, combine the mustard, beer, vinegar and honey. Bring to a boil, reduce to a simmer and cook until slightly thickened. Season to taste with salt and pepper.
In a large sauté pan over medium heat, melt the butter. Add the onions, sugar and salt and sauté about 40 minutes, until onions are very soft and browned.
Grill sausages over medium heat until heated through, about 5-8 minutes.
Generously brush sausage with mustard as they cook.
Place the rolls on the grill and toast on each side. To assemble, brush each side of the rolls with the honey mustard. Add and sausage link and top with caramelized onions. Serve warm.