Prepare parsnips: Trim tops and bottoms; peel, and slice in half crosswise to separate thick and thin parts. Halve or quarter thick parts lengthwise, until all parsnip pieces are roughly the same size. (In other words, slice 'em into matchsticks.).
Prepare sweet potatoes: Trim ends, then quarter lengthwise and cut diagonally into 1/2" pieces.
On a large rimmed baking sheet, toss parsnips and sweet potatoes with oil; season generously with salt and pepper. Spread in a single layer.
Roast until tender and golden, about 30 minutes. (Toss once or twice while roasting.) When done, transfer to a large bowl.
In a small bowl, stir together maple syrup and mustard. Pour over parsnips and sweet potatoes, and stir to coat. Serve immediately.