In a large bowl, combine the sliced onions, minced garlic, lemon juice, white vinegar, vegetable oil, Dijon mustard, black pepper, salt, and minced Scotch bonnet peppers (if using).
Add the chicken pieces to the marinade, ensuring they are well coated. Cover and refrigerate for at least 2 hours, or overnight for best results.
Preheat your oven to 375°F (190°C).
Remove the chicken from the marinade, reserving the marinade for later use.
In a large oven-safe skillet or Dutch oven, heat a small amount of vegetable oil over medium heat.
Brown the chicken pieces on all sides, about 5-7 minutes per side. Remove the chicken and set aside.
In the same skillet or Dutch oven, add the reserved marinade with the onions and cook over medium heat until the onions are soft and caramelized, about 10-15 minutes.
Add the chicken broth or water to the skillet with the onions and bring to a simmer.
Return the browned chicken pieces to the skillet, nestling them into the onions.
Cover the skillet and transfer it to the preheated oven.
Bake for about 30-40 minutes, or until the chicken is cooked through and tender.
Remove the skillet from the oven and let the Yassa cool slightly.
Enjoy your meal!