Prep the ingredients per the instructions above.
Place the potatoes into a large bowl and cover completely with cold water for 20 minutes. Drain well and pat dry.
Preheat an airfryer to 400°F (204°C). Cook the fries according to the directions on the air fryer. Season with salt immediately to taste.
Heat the oil in a skillet over medium heat. Saute the onion in the skillet. Season with salt and pepper. Cook for 20 minutes, stirring occasionally. When the onion becomes soft and golden, set aside.
Preheat the oven to 400°F (204°C).
Cook the kielbasa on a baking sheet in the oven to an internal temperature of 160°F (71°C).
To build the sandwich, place a sausage on a bun, add yellow mustard, and top with caramelized onion and 2-3 pickled peppers, if desired. Wrap the sandwich in deli paper, parchment paper, or foil and leave for 5 minutes to get steamed up.
Enjoy Maxwell Street Polish sandwiches with fries, ketchup, and kombucha.