Chop unpeeled potatoes and onions into large (1 inch sized) pieces.
Divide chopped potatoes and onions between 2 large aluminum roasting pans. Pour 1/4 cup oil over each pan of vegetables. Toss the vegetables to coat them with the oil.
Sprinkle each pan of potatoes with 2 tablespoons kosher salt, 2 tablespoons rosemary, 2 tablespoons garlic powder and 1/2 teaspoon pepper. Toss again to distribute seasonings. Roast at 350 degrees for an hour. Stir the vegetables two or three times during the hour. The vegetables are done when an inserted fork meets with no resistance.
Can be made up to 2 days in advance, covered, and refrigerated. When ready to serve: reheat, UNCOVERED, in a roaster oven on high until hot (about 15 - 30 minutes), stirring several times - - - then REDUCE HEAT to low to hold for up to 2 hours.