Preheat oven to 400°F. Grease an extra-large, rimmed baking sheet or spray with nonstick cooking spray. You can line the sheet pan with heavy duty aluminum foil or parchment paper before greasing for easy cleanup, if desired.
In a large bowl, whisk together the dry Italian dressing mix, ¼ cup of the olive oil, garlic powder, and black pepper. Add the potatoes and sausage; toss to coat.
Spread the potatoes, sausage, and any extra olive oil mixture in a single layer on the prepared baking sheet. Push them to the side to make some room for the cabbage on the tray as well. Brush or rub the cabbage with the remaining 1 tablespoon of olive oil. Season the cabbage with salt and pepper to taste.
Roast the potatoes, sausage, and cabbage in the preheated oven until the potatoes are tender, the sausage is nicely browned, and the cabbage is tender and golden on the edges, about 30-35 minutes, flipping halfway through. The cabbage might fall apart a bit when you turn it, but that’s fine!
Garnish with chopped fresh parsley, if desired.