Preheat the oven to 400ºF. In a small bowl whisk together the olive oil, red wine vinegar, mustard, crushed garlic, salt, and freshly cracked pepper for the vinaigrette. Set the vinaigrette aside.
Slice the kielbasa into 1/4-inch thick rounds. Wash the potatoes well and slice them into 1/4-inch rounds as well. Place the kielbasa and potatoes on a large baking sheet and drizzle with 1 Tbsp olive oil. Toss the kielbasa and potatoes in the oil until they are well coated and the surface of the baking sheet is also covered in oil. Sprinkle a pinch of salt and pepper over top.
Remove any dirty or damaged leaves from the cabbage. Cut the stem off the cabbage, then cut it in half. Reserve one half for a different recipe. Slice the remaining half into 1-inch wide slices. Cut each slice into two pieces. Place the cabbage pieces on the baking sheet with the kielbasa and potatoes, nestling them down so that they are laying flat on the baking sheet.
Brush the remaining 1 Tbsp olive oil over the surface of the cabbage pieces and add a final pinch of salt and pepper to each.
Roast the kielbasa, potatoes, and cabbage in the preheated oven for 20 minutes. Remove the baking sheet from the oven and carefully flip the kielbasa, potatoes, and cabbage pieces. The cabbage may fall apart a bit as it's flipped, which is okay. #Return the baking sheet to the oven and roast for an additional 10-15 minutes, or until the cabbage is tender and the edges are slightly brown and crispy. The kielbasa and potato slices should be well browned.
Remove the baking sheet from the oven and top with fresh chopped parsley and a drizzle of the mustard vinaigrette. Serve warm.