Preheat the oven to 425°F. Line a large baking sheet with parchment paper or foil.
Place each kielbasa between two chopsticks or wooden skewers. Using a sharp knife, slice into ¼-inch cuts, stopping at the chopsticks so you don’t cut all the way through.
Slice the kielbasa crosswise into 1-inch bite-sized pieces (about 3 rings per bite). You should get about 20 bites per ring, or roughly 40 total.
In a small bowl, whisk together the Dijon mustard, apple cider vinegar, brown sugar, and Worcestershire sauce until smooth.
Divide the mustard mixture evenly into two bowls.
To one bowl, add the mayonnaise and stir until combined. Cover and refrigerate to use as the dipping sauce.
Arrange the kielbasa bites in a single layer on the prepared baking sheet. Brush or spoon about half of the remaining mustard sauce over the bites, making sure it gets between the cuts. Reserve the rest of the sauce.
Bake for 18–22 minutes, flipping the bites halfway through, until browned and caramelized.
Remove from the oven and brush with the remaining mustard sauce. Garnish with chopped parsley if desired.
Serve warm with the chilled creamy Dijon dipping sauce.