Marinate the chicken:
In a large zip-top bag or mixing bowl, combine the chicken with oil and your spices. Toss well to coat, then set aside to marinate while you prep the vegetables.
Caramelize the onions:
Heat a drizzle of oil in a large pan over medium-low heat. Add the onions and cook, stirring occasionally, until deeply golden and caramelized, about 30 minutes.
Cook the vegetables:
Once the onions are caramelized, add the leeks, cubed squash, and dates. Sauté together for a few minutes, just until everything is lightly softened and coated in the onion mixture.
Preheat the oven to 350°F. Spread the vegetable mixture across the bottom of a large roasting pan. Place the marinated chicken on top. Drizzle with honey or silan.
Bake:
Cover the pan with parchment paper, then foil, and bake for 1 hour. Remove the foil and parchment, then bake uncovered for an additional 30 minutes, until the chicken is cooked through and beautifully browned.
Serve:
Transfer the chicken to a serving platter and spoon the vegetables (and all the pan juices!) over the top. Serve with couscous for a sweet-and-savory dish that’s perfect for Rosh Hashanah.