Preheat oven to 425°. Cut off fennel stalks and reserve feathery leaves. Trim base of bulb and any tough outer layers. Halve bulb lengthwise, cut out core in a V, and cut bulb lengthwise into slim wedges.
Oil a large rimmed baking sheet and arrange sweet potatoes and fennel in a single layer. Sprinkle with 1/2 tsp. salt and 1/4 tsp. pepper. Arrange onion and apples over potatoes and sprinkle with 1/4 tsp. each salt and pepper. Set chicken on top. In a small bowl, combine butter, honey, five-spice, and remaining 1/2 tsp. salt. Brush onto chicken.
Roast 15 minutes, then tip pan and baste ingredients with juices (use a bulb baster or wide spoon). Continue to roast until chicken is nicely browned and no longer pink at bone (cut to test), 15 to 20 minutes more. Baste everything again and scatter reserved fennel leaves on top.