Preheat oven to 400°. Coat a large rimmed baking sheet with cooking spray.
Scrub potatoes with vegetable brush under cold running water. Cut into ¼-inch wedges and transfer to a large bowl. Add the fennel, onions, garlic, chicken, oil, rosemary, fennel seeds, salt and pepper and toss to mix. Lift out the chicken and set aside.
Arrange potato mixture evenly on the prepared baking sheet. Arrange chicken on top of the potato mixture. Roast 30 minutes. Remove from oven, stir the vegetables. Return to oven and cook until vegetables are browned and tender and an instant-read thermometer inserted into the thickest portion of the thigh registers 175°, about 20 minutes longer.
Discard chicken skin before serving.
Nutritional Analysis per serving: 325 calories, 24 g protein, 33 g carbohydrates, 12 g fat, 3 g saturated fat, (31% of calories from fat), 69 mg cholesterol, 6 g fiber, 562 mg sodium.