Position 1 rack in top third of oven and 1 rack in bottom third of oven and preheat to 425°F. Spray 2 large rimmed baking sheets with nonstick spray. Whisk mustard, olive oil, butter, lemon juice, garlic, oregano, lemon peel, and salt in large bowl to blend. Add potatoes; sprinkle generously with freshly ground black pepper and toss to coat. Divide potatoes between prepared baking sheets, leaving any excess mustard mixture behind in bowl. Spread potatoes in single layer. Roast potatoes 20 minutes. Reverse baking sheets and roast until potatoes are crusty outside and tender inside, turning occasionally, about 25 minutes longer.
Transfer potatoes to serving bowl.
Do ahead: Can be made 2 hours ahead. Let stand on baking sheets at room temperature. Rewarm potatoes in 425°F oven 10 minutes.
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