Make the onions. In a large nonstick skillet, heat the oil over medium heat. Add the onions and let them sit for a few minutes to develop some color, then stir. Continue cooking, stirring occasionally, until the onions are browned and translucent, but not quite caramelized. About 20 minutes.
Remove the onions to a bowl and cover with foil to keep warm. In the same pan that the onions were cooked in, heat the additional 2 tablespoons of oil and add the sausages. Working in batches, cook the sausages for 5 minutes on each side, until browned and crisp.
Place the buns on a plate and cover with a damp paper towel. Microwave for 15-20 seconds until warmed and steamy.
Add a sausage to each bun and top with the onions, yellow mustard and sport peppers, if using.