One day before: Cut the onions into thin slices. To make the marinade, combine onions, mustard, lemon juice, chile pepper, salt, and pepper in a large bowl. Place the pieces of chicken into the marinade, cover tightly with plastic wrap, and refrigerate.
The day of: Remove the chicken pieces from the marinade and grill until lightly browned on both sides. Meanwhile, in a large, high-sided skillet or Dutch oven, sauté the onion slices until translucent. Next, add the grilled chicken pieces along with the rest of the marinade to the onions and simmer over medium heat until chicken is tender, about 20-30 minutes. Serve with the marinade as a sauce.