preheat oven to 425°. Toss potatoes with oil; season with salt and black pepper. Transfer potatoes to a baking sheet; roast until Golden, 20 to 25 minutes.
Heat butter in a pan over medium low. Add onion and garlic; cook until onion is translucent, 5 to 7 minutes. Whisk in flour, and Mace; cook 1 to 2 minutes whisking constantly.
Whisk in milk and increase heat to medium. Bring mixture to a simmer and cook until thickened, 3 to 4 minutes. Add peas; cook 1 to 2 minutes more. Stir in dill, zest and lemon juice. Season peas with salt and cayenne and serve over roasted potatoes. Serves six.