Preheat the oven to 350°F. Cut the potatoes into large 2-inch chunks.
Mix the olive oil, mustard, melted butter, garlic, lemon zest, lemon juice, rosemary leaves, and salt and pepper in a large bowl.
Add the potatoes and, using a spoon, coat them with the mustard mix.
Spread the potatoes out on a baking sheet. Roast in the oven for 40 minutes or until the potatoes are crispy on the outside and soft and fluffy on the inside.