Marinate the Chicken
In a large bowl or zip-top bag, combine chicken, lemon juice, mustard, garlic, ginger, pepper, and salt. Add half of the sliced onions. Toss to coat, cover, and marinate in the refrigerator for at least 2 hours or overnight for best flavor.
Sear the Chicken
Remove chicken from marinade and pat dry (reserve marinade). Heat oil in a large skillet or Dutch oven over medium-high heat. Brown chicken pieces on all sides until golden, about 5–6 minutes per side. Transfer to a plate.
Cook the Onions
In the same skillet, add all the onions (marinated and fresh). Cook on medium heat, stirring often, until deeply golden and caramelized — about 20 minutes.
Simmer the Dish
Return chicken to the pan with onions. Add reserved marinade, hot pepper, and broth. Cover and simmer over medium-low heat for 25–30 minutes, until chicken is cooked through and tender.
Finish and Serve
Remove the hot pepper. Taste and adjust seasoning. Serve chicken and onion sauce over warm rice.