Preheat oven to 425°F and line a rimmed sheet pan with foil; pat chicken thighs very dry and season both sides with 1 teaspoon kosher salt and 1/4 teaspoon black pepper, rubbing some seasoning under the skin where possible.

If searing, heat 1 tablespoon oil in a 12-inch skillet over medium-high heat and sear thighs skin-side down until golden, 3–4 minutes, then transfer skin-side up to the sheet pan; searing is optional but improves color.

Roast the chicken in the preheated oven for 20 minutes to render fat and start browning.

While chicken roasts, combine sliced onion, orange juice, cranberries, 1 tablespoon chopped rosemary, minced garlic, orange zest, remaining 1 tablespoon oil, 1/2 teaspoon kosher salt, and remaining 1/2 teaspoon black pepper in a bowl; add maple syrup only if using.

Remove the pan after 20 minutes and spoon the cranberry-rosemary mixture around and between the thighs (do not pile on top of the skin). Stir any pan juices into the fruit mixture, then return to the oven and roast another 12–15 minutes until the thickest part of a thigh reads 165°F and cranberries are softened.

Let the chicken rest on the pan 5 minutes, then spoon pan juices and cranberries over the thighs, garnish with remaining chopped rosemary, and serve.