Preheat oven to 400 degrees. Line a baking sheet with foil.
In a large bowl, add onions, peppers, olive oil, balsamic vinegar, a little salt, pepper, and garlic powder. Toss well to be sure everything is coated with oil and seasoned evenly. Pour onto the prepared baking sheet and place in the center of the oven. Bake for 15 minutes.
Place the sausage on a cutting board and using a sharp knife score it into thin even layers. Be sure to not cut each slice all the way through, just halfway through. You want the complete sausage in tack.
In a small bowl mix together the bbq sauce and mustard. Spread the sauce over the top of the sausage. Add the sausage to the baking sheet with vegetables and bake for 30-35 minutes until onions and peppers are well cooked.
To serve: cut the sausage into 4 servings. Divide the onions and peppers among 4 servings and top each with the sausage. Any leftovers freeze great!