Add your chicken to a bowl along with salt, pepper, and the whole grain mustard. Toss to combine and allow to marinate for 30 minutes.
In a separate bowl add your plums and onions. Season with salt, pepper, allspice, and rosemary. Drizzle with olive oil and toss to evenly coat.
Position the chicken on a baking sheet. Roast for 30 minutes.
Add the plums and onions to the baking sheet along with extra rosemary. Roast for another 20 minutes, or until the internal temperature of the chicken is 165 degrees.
Serve with rice, mashed potatoes, or polenta for a complete meal.