Heat the oven to 425°F. Finely grate 1 tsp. orange zest. Stir together the zest, oil, 1 tsp. salt, and the chile flakes in a small bowl.
On one end of a large rimmed baking sheet, toss 1 Tbs. of the oil mixture with the potatoes, onions, and 1 sprig rosemary; separate the onions into rings, and spread the onions and potatoes into a single layer as much as possible. At the other end of the baking sheet, arrange the chicken skin side up and brush the tops of the chicken thighs with the remaining oil mixture. Tuck the remaining rosemary sprig between a couple of thighs and sprinkle the thighs and vegetables lightly with salt.
Roast for 20 min. Baste the chicken with the pan drippings using a brush and stir the potatoes and onions. Continue to roast, basting and stirring every 10 min., until the chicken skin looks crisp and golden and the potatoes are lightly browned in spots, about 30 min. more.
Meanwhile, peel the oranges with a sharp knife, making sure you’ve removed the pith and membrane. Slice crosswise into roughly 1/2-inch circles and then chop into roughly 1/2-inch pieces, discarding any thick center membranes. Transfer to a small bowl and stir in 1/4 tsp. of the minced rosemary.
When the chicken is done, remove the rosemary sprigs from the pan and discard. Stir the potatoes and onions, transfer with a slotted spoon to a serving bowl, and stir in the remaining 1/2 tsp. minced rosemary. Baste the chicken and transfer with tongs to a serving platter, top with the orange mixture, and serve hot.
Try a youthful, fruity Pinot Noir from Oregon or California. Good choices include the 2002 Chehalem “Three Vineyards” from Oregon or the 2003 Hahn Estates Pinot, Monterey ($14).