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Position 1 rack in top third of oven and 1 rack in bottom third of oven and preheat to 425°F.
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Grease a large rimmed baking sheet with oil.
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Whisk mustard, olive oil, butter, lemon juice, garlic, oregano, lemon peel, and salt in large bowl to blend.
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Add potatoes
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Sprinkle generously with freshly ground black pepper
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and toss to coat.
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Place potatoes on prepared baking sheet, leaving any excess mustard mixture behind in bowl.
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Spread potatoes in single layer.
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Roast potatoes 20 minutes.
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Reverse baking sheets and roast until potatoes are crusty outside and tender inside, turning occasionally, about 25 minutes longer.