Slice the peppers and onion into 1-2 inch long slices. Slice the garlic. Measure out the mustards and vinegar and add them to prep bowls.
Parboil the Sausage
Add the sausage to a medium-sized skillet and add water until the sausage is half covered. Bring to a boil and cook for 5-7 minutes. Remove from pan.
Cook the Peppers and Onions
Add about a tablespoon of sunflower oil and heat the Blackstone or skillet to medium-high. Add the peppers and onions and season with kosher salt and pepper. Cook until they start to char and become soft, about 10 minutes, stirring regularly.
Grill the Sausage
While the peppers and onions are cooking, add the sausage to the grill at medium-high. Turn often and cook for about 10 minutes. Remove when done.
Back to the Peppers and Onions
Once the peppers and onions are nicely charred and softer, add the garlic and cook until it is softened and a bit charred, about 3-5 minutes. Reduce the heat and add the mustards, horseradish, and balsamic vinegar. Stir to combine with the veggies and let cook for about 3-5 minutes.
Serve and Enjoy!
Leave the sausage whole, or slice the sausage into bite-sized pieces and mix with the veggies. Serve by itself, over pasta, or with other side dishes. Or add the sausage to a gluten-free hot dog bun and cover with the veggies. Top with mustard if desired. Enjoy!