Pat the chicken thighs dry with paper towels and place them in a large non-reactive bowl.
Add the sliced onions, minced garlic, Dijon mustard, lemon juice, smoked paprika, cayenne pepper, bouillon powder, and salt.
Toss thoroughly so the chicken and onions are evenly coated with the marinade.
Cover and refrigerate for at least 2 hours. For deeper flavor, marinate the chicken overnight.
Remove the chicken from the refrigerator. Lift each thigh from the marinade and gently scrape the onions back into the bowl. Reserve all of the marinade and onions.
Heat the olive oil in a large skillet or Dutch oven over medium-high heat.
Add the chicken thighs and brown for approximately 4–5 minutes per side. They don't need to be completely cooked at this stage.
Transfer the browned chicken to a clean plate.
Add the reserved onions and marinade to the same pan.
Reduce the heat to medium-low and cook the onions for about 5–10 minutes, stirring regularly, until softened and beginning to caramelize.
Return the browned chicken thighs to the pan, nestling them into the onions.
Pour in the chicken broth and gently stir the sauce around the chicken.
Simmer the Poulet Yassa Recipe for approximately 15–20 minutes, or until the chicken is thoroughly cooked and reaches an internal temperature of 165°F (74°C).
Continue simmering for a few additional minutes if you prefer a thicker, more concentrated onion sauce.
Taste and adjust the salt, cayenne, or lemon juice as desired.
Spoon the caramelized onion sauce generously over the chicken and serve hot with rice.