In a medium saucepan, heat the butter over medium heat until melted. Add the beer and onions and cook, stirring often and reducing the heat to low as the onions start to brown, until the onions are caramelized and all of the liquid has reduced to allow the sugars to brown, 5 to 7 minutes. Season with salt and pepper and set aside.
In a large saute pan, heat the canola oil over medium heat. Split the links through the center just to open them up so they lay flat. Sear the links on the cut side until brown, about 5 minutes, then remove them from the pan. In the same pan, toast the buns again on the cut side until brown, a few minutes.
Spread the mustard onto the buns, then add the links. Top with the pickled peppers and caramelized onions. Finish with a small handful of watercress and serve.