Preheat the oven to 425°F. Grease a large baking pan or roasting pan with cooking spray.
Wash and cut potatoes into 1½-inch chunks. Pat completely dry with paper towels.
Peel and slice onions into half-rings, about ¼-inch thick.
In a large bowl, whisk together the melted butter, olive oil, garlic powder, onion powder, smoked paprika, Italian seasoning, salt, and black pepper.
Add the potatoes and onions to the bowl and toss until every piece is evenly coated.
Spread in a single layer in the prepared baking pan. Do not overlap or pile.
Roast for 25 minutes, then remove from the oven and stir and flip the potatoes.
Return to the oven and roast for another 18 to 20 minutes, until golden and fork-tender and the onions are caramelized.
Remove from the oven. Serve hot, garnished with fresh chopped parsley if desired.