Place a rimmed baking sheet in the upper third of the oven and preheat to 500 degrees F. Whisk the orange juice, honey, mustard, olive oil, 1 teaspoon salt and a few grinds of pepper in a large bowl. Add the chicken, sliced oranges, sliced fennel, red onion, potatoes, thyme and fennel seeds; toss to coat.
Remove the hot baking sheet from the oven and add the vegetables in a single layer. Arrange the chicken skin-side up on top of the vegetables; season with salt and pepper. Return to the oven and roast until the chicken is golden and cooked through and the vegetables are tender, about 30 minutes. Top with fennel fronds.