In a large bowl, combine the chicken pieces, sliced onions, minced garlic, Dijon mustard, vegetable oil, vinegar, lemon juice, water, minced peppers (if using), soy sauce, fresh thyme, bay leaves, salt, and pepper.
Mix well to coat the chicken thoroughly. Cover and marinate in the refrigerator for at least 2 hours, preferably overnight for best results.
Remove the chicken pieces from the marinade, shaking off any excess onions. Reserve the marinade.
In a large skillet or Dutch oven, heat a little vegetable oil over medium-high heat. Add the chicken pieces and brown them on all sides, working in batches if necessary. Remove the browned chicken and set aside.
In the same skillet, add the reserved marinade, including the onions and all the liquid. Cook over medium heat, stirring occasionally, until the onions are soft and translucent, about 10-15 minutes.
Return the browned chicken pieces to the skillet with the onions. Stir to combine.
Reduce the heat to low, cover, and simmer for about 45 minutes to 1 hour, or until the chicken is cooked through and tender. Stir occasionally and add a little water if needed to prevent sticking.
Enjoy your meal!