Preheat your oven to 400°F (200°C). Line a baking sheet with parchment paper or aluminum foil for easy cleanup.

Prepare the kielbasa: Place the sausage on a cutting board. Using a sharp knife, make thin, parallel slices across the top of the sausage, going about halfway through, without cutting all the way to the bottom. Space the cuts about 1/4 inch apart—this creates the Hasselback effect.

Mix the glaze: In a small bowl, whisk together the Dijon mustard, brown sugar, apple cider vinegar, garlic powder, and black pepper until smooth.

Brush the glaze generously over the scored kielbasa, ensuring some of the glaze seeps down into the slices.

Bake for 20-25 minutes, or until the sausage is heated through and the edges of the slices are slightly curled and caramelized. If desired, turn on the broiler for the last 1-2 minutes to achieve extra crispiness, watching carefully to prevent burning.

Remove from the oven. If using, immediately sprinkle the tops lightly with Everything Bagel Seasoning while hot. Let cool for 5 minutes before slicing into individual 'bites' between the cuts.