Preheat the oven to 425 degrees.
Cut the potatoes in halves or quarters, depending on their size, and place them on a sheet pan. Remove the ends of the onions, peel them, and cut them in half. Slice them crosswise in 1/4-inch-thick slices to make half-rounds. Toss the onions and potatoes together on the sheet pan. Add the olive oil, mustard, 2 teaspoons salt, and the pepper and toss them together. Bake for 50 minutes to 1 hour, until the potatoes are lightly browned on the outside and tender on the inside. Toss the potatoes from time to time with a metal spatula so they brown evenly.
Serve hot sprinkled with chopped parsley and a little extra salt.
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