Toss the parsnips with 1 tablespoon of the olive oil and 1 teaspoon of the salt on
a rimmed baking sheet; arrange in a single layer. Toss the sweet potatoes with 1
tablespoon of the olive oil and the remaining 1 teaspoon salt; arrange in a single
layer on a separate rimmed baking sheet. Roast the parsnips for 10 minutes;
remove and gently stir. Then place the parsnips back in the oven and add the
sheet of sweet potatoes. Roast the vegetables, stirring once halfway through,
until tender and lightly browned, 40 to 50 minutes more.
Meanwhile, in a small bowl, whisk together the olives, the remaining 3
tablespoons olive oil, the lemon juice, shallot, and garlic. When ready to serve,
transfer the vegetables to a platter and spoon the dressing on top. Sprinkle with
the mint and coriander. Serve warm or at room temperature.