Wash and pat the chicken thighs dry, then place them in a large bowl.
Add the sliced onions, minced garlic, lemon juice, Dijon mustard, smoked paprika, cayenne pepper, chicken bouillon powder, and salt.
Mix thoroughly so the chicken is coated with the marinade and seasonings.
Cover and refrigerate for at least 2 hours, or preferably overnight for deeper flavor.
Remove the chicken from the marinade. Scrape off the onions and marinade from the chicken, but reserve everything for later.
Heat the olive oil in a large skillet or Dutch oven over medium-high heat.
Add the chicken thighs and brown for about 4–5 minutes on each side.
Remove the browned chicken from the pan and set aside.
Add the reserved onions and marinade to the same pan. Cook over low to medium-low heat for about 5 minutes, stirring occasionally, until the onions begin to soften and caramelize.
Return the browned chicken to the pan and pour in the chicken broth.
Cover and simmer for 15–20 minutes, or until the chicken is completely cooked and tender. This is where the Poulet Yassa develops its rich, tangy onion sauce.
Taste the sauce and adjust the salt if needed.
Serve hot with fluffy rice and plenty of the caramelized onion sauce.