If you're not familiar with parsnips, they look like white carrots and have a decidedly sweet, earthy flavor. Avoid larger parsnips, which tend to have tough, woody cores.
Place a rimmed baking sheet in oven; preheat oven to 500°F. (Do not remove pan while oven preheats.)
Combine parsnips, walnuts, thyme, maple syrup, salt, and pepper. Arrange in a single layer on preheated baking sheet. Bake in preheated oven until tender, about 10 minutes. Toss with vinegar.