Preheat your oven to 400°F (200°C) and line a large, rimmed baking sheet with parchment paper for easy cleanup.
In a large bowl, combine the kielbasa rounds, sweet potato cubes, apple chunks, Brussels sprout halves, and red onion chunks. Drizzle with 3 tablespoons of olive oil and season with 1 teaspoon kosher salt and 1/2 teaspoon black pepper. Toss everything together until the pieces are evenly coated in oil and seasoning.
Spread the sausage and vegetable mixture into a single, even layer on the prepared baking sheet, ensuring nothing is crowded to allow for proper roasting and browning.
Roast on the center oven rack for 25-30 minutes, stirring halfway through, until the sweet potatoes are tender when pierced with a fork and the sausage and Brussels sprouts have browned edges.
While the sheet pan bakes, whisk together 1/2 cup mayonnaise, 2 tablespoons Dijon mustard, 1 tablespoon maple syrup, and 1 teaspoon curry powder in a small bowl until a smooth, creamy sauce forms.
Once the bake is finished, remove the pan from the oven. Drizzle the creamy curry Dijon sauce over the top or serve it on the side for dipping. Garnish with 1 tablespoon of fresh chopped parsley before serving.