Tender oven-roasted beets and potatoes tossed with toasted walnuts make an ideal picnic side dish.
Preheat oven to 450 degrees, with racks in top and middle. Place beets on a large piece of foil on a baking sheet. Fold foil around beets and crimp ends to form a packet. Cook beets on sheet on middle rack, 30 minutes.
On a rimmed baking sheet, toss potatoes with oil and season with salt and pepper. Arrange potatoes, cut side down, on sheet. After beets have cooked 30 minutes, place potatoes on top rack. Cook 15 minutes. Flip potatoes and sprinkle with walnuts (if using). Cook until walnuts are toasted, potatoes are golden, and beets are tender when pierced with a knife, 5 to 10 minutes.
Remove beets from foil and let cool. Transfer potatoes and walnuts to a large bowl. Rub beets with a paper towel to remove skin and cut each into 4 to 6 wedges, depending on size. Toss beets with potatoes, walnuts, and chives and season to taste with vinegar.