In a small bowl, toss the prunes with the vinegar and brown sugar. Let marinate for at least 10 minutes.
Meanwhile, season the chicken thighs with salt and pepper. In a large skillet or dutch oven, heat the oil over medium-high heat. Add the chicken and cook, turning once, until browned, about 8 minutes; transfer to a plate.
Discard all but 1 tablespoon of fat and lower the heat to medium. Add the onion and cook, stirring, until soft and golden, about 5 minutes. Add the prune-vinegar mixture and return the chicken thighs to the pan. Add the chicken broth and simmer until the chicken is cooked through and the liquid is reduced by half, about 20 minutes. Stir in the cream.
Divide the rice and chicken thighs between 2 plates and spoon the sauce on top.