Heat a small skillet over medium heat. Add the fennel seeds and toast until fragrant, about 2 minutes — don't let them burn.
Crush the toasted seeds coarsely using a mortar and pestle. No mortar and pestle? A spice grinder works well, or crush them under a heavy pan or the flat side of a knife on a cutting board. Add the crushed fennel to a large bowl, big enough to hold all the chicken. Stir in the orange juice, orange zest, garlic, allspice, red pepper flakes, and honey. Season the chicken with salt and pepper, then add it to the bowl. Turn the pieces with a spatula to coat them evenly in the marinade. Add the plum slices and thyme sprigs. Give everything a quick stir, cover, and refrigerate for at least 2 hours, up to 24.
When ready to cook, preheat the oven to 425°F.
Arrange the chicken on a rimmed sheet pan coated with oil. Tuck the plums, onion wedges, and potatoes in between the pieces.
Spoon any remaining marinade liquid evenly over the chicken.
Drizzle with a little more olive oil and season with salt and pepper.
Roast 45 minutes, or until the chicken thighs are done.
To serve, plate the chicken and spoon the plums, onions, and potatoes around it.
Spoon any pan juices over the top, garnish with torn basil, and serve.