Preheat your oven to 425°F (220°C), or preheat your grill to medium-high heat (around 400°F). If using an oven, line a large baking sheet with foil for easy transport.

Slice the kielbasa into ½-inch coins and the potatoes into ¼-inch thick rounds or halves if using baby potatoes. For faster, more even cooking, microwave the potato slices for 3-4 minutes until just starting to soften, then set aside.

In a large bowl, combine the potatoes, kielbasa, olive oil, minced garlic, smoked paprika, onion powder, black pepper, and salt. Toss everything together until evenly coated.

In a small bowl, mix the spicy brown mustard with 2 tablespoons of softened butter until combined into a paste. This will be distributed among the packets for maximum flavor impact.

Lay out 4 sheets of heavy-duty foil (18 inches each), doubled up for strength. Lightly spray the center of each with cooking spray.

Divide the kielbasa and potato mixture evenly among the 4 foil sheets, piling it in the center. Top each with a generous spoonful of the mustard butter mixture and one additional pat of plain butter.

Fold the foil up and over the filling, bringing the two long sides together and folding them down tightly to create a sealed seam. Fold in the short ends to fully seal the packet. The goal is a tightly closed pouch that traps steam.

OVEN: Place packets on the prepared baking sheet and bake at 425°F for 28-32 minutes, until potatoes are fork-tender and the sausage is heated through and slightly caramelized.

GRILL: Place packets seam-side up on the grill grates over medium-high heat. Cook for 15 minutes, then carefully flip and cook another 12-15 minutes. For charred edges, carefully open the top of each packet in the last 5 minutes.

CAMPFIRE: Place packets on hot coals (not open flames). Cook for 20-25 minutes, turning once halfway through.

Carefully open each packet away from you — the steam is very hot. If adding cheese, sprinkle it on now, fold the foil back loosely, and wait 3-5 minutes for it to melt.

Serve directly in the foil packet for easy cleanup, or transfer to plates. Garnish with fresh chopped parsley and an extra drizzle of spicy mustard if desired.