In a large bowl, combine half of the sliced onions, minced garlic, Dijon mustard, lemon juice, white vinegar, black pepper, and salt. Mix well.

Add the chicken pieces to the marinade, ensuring they are fully coated. Cover the bowl and refrigerate for at least 2 hours, preferably 4 hours or overnight.

When ready to cook, remove the chicken from the marinade, reserving the marinade and onions. Pat the chicken dry with paper towels.

Heat 2 tablespoons of vegetable oil in a large Dutch oven or pot over medium-high heat. Sear the chicken pieces in batches until golden brown on all sides. Remove the chicken and set aside.

Add the remaining 1 tablespoon of oil to the pot, along with the other half of the fresh onions and the reserved marinated onions (without the chicken). Reduce heat to medium-low. Cook slowly, stirring occasionally, until the onions are deeply caramelized and tender, about 20-30 minutes. Be patient, as this step builds flavor.

Return the seared chicken to the pot with the caramelized onions. Add the reserved marinade, bay leaves, and chicken broth (or water). If using, add the whole or pierced scotch bonnet pepper.

Bring to a gentle simmer, then reduce heat to low, cover, and cook for 45-60 minutes, or until the chicken is fork-tender and cooked through. Stir occasionally to prevent sticking.

Taste and adjust seasoning if necessary. Remove the scotch bonnet pepper and bay leaves before serving if you prefer less heat.

Serve hot with white rice or couscous.