Yassa is defined by the massive amount of onion. Do not be afraid of this quantity, as long cooking breaks down the onion and caramelizes it into a thick, sweet-and-sour sauce that is the foundation of the entire dish.
Traditionally, bone-in chicken is used, which adds more flavour to the sauce, but boneless chicken thighs or breasts also work well if you prefer a quicker preparation. The sour taste is achieved solely with lemon juice, not vinegar.
In a large bowl, mix the lemon juice, mustard, garlic, salt, and pepper.
Add half of the chopped onion and the chicken. Thoroughly mix the meat with the marinade so that it is fully coated.
Cover the bowl and let it marinate in the refrigerator for at least 1 hour (ideally 4 hours or overnight).
Remove the marinated chicken pieces (without the onion) from the marinade and pat them dry.
In a deep pot or pan, heat the oil over medium-high heat.
Sear the meat on all sides until golden brown. Remove the seared chicken and set aside.
Add the remaining half of the fresh onion and the onion left over from the marinade to the pot (where the chicken was fried). Add a tablespoon of oil if necessary.
Stew the onion over medium to low heat for about 20–30 minutes, stirring occasionally, until it completely softens and caramelizes to a golden-brown colour. This is an important step for flavour depth.
Return the seared chicken to the caramelized onion. Add the bay leaf, a small cup of water or broth, and the optional chili pepper.
Bring to a boil, then reduce the heat, cover, and simmer for 30 minutes until the chicken is cooked through and the sauce is thick and rich.
Taste and adjust the salt or add more lemon juice for increased acidity if needed.
Remove the bay leaf and chili. Serve hot, topped with the rich onion sauce, over a mound of cooked white rice.