Make the aioli. In a food processor, blend the egg on high for 20 seconds.  Add the ground mustard, vinegar, and salt.  Cover and blend on high speed for another 20 seconds. Scrape down the sides of the food processor.  On low speed, add a few drops of oil at a time until the mixture thickens.  Increase the speed to high and add the oil in a very slow, steady stream.  Scrape down the sides of the food processor again.  Add lemon juice and salt to state and blend on high speed for 5 to 10 seconds to incorporate.

Prep.  Preheat the grill to medium-high heat.  Slice the peppers and onions.  Drizzle with oil and season with salt and pepper.  Heat cast iron pan on one side of the grill.

Grill. Add the peppers and onions to the cast iron pan and cook for 10 minutes, stirring occasionally.  Grill the sausages for 5 minutes each side over medium heat.  Just before the sausages and peppers are done, toast the buns.  Optional: Spread the truffle aioli on inside of the buns, open, and toast face down or toast the buns plain.

Make the sandwiches.  Stuff each bun with peppers and onions, top with grilled sausage and drizzle with truffle aioli.