In a large bowl, combine fresh lemon juice, Dijon mustard, minced garlic, salt, and pepper. Whisk until well blended. Add the chicken, ensuring each piece is thoroughly coated. Cover the bowl with plastic wrap and refrigerate for at least 4 hours, preferably overnight.

Heat a heavy-bottomed pot or Dutch oven over medium-high heat and add a couple of tablespoons of cooking oil. Once shimmering, remove the chicken from the marinade (reserving the marinade) and carefully place it in the pot. Sear the chicken for 5–7 minutes on each side until it is golden brown. Transfer the seared chicken to a plate and set it aside to rest.

In the same pot, add the sliced onions and reduce the heat to medium-low. Cook the onions, stirring occasionally, for about 20-25 minutes until they soften and turn a deep golden brown.

Once the onions are caramelized, pour in the reserved marinade along with bay leaves and chicken stock. Stir well to deglaze the pot, scraping up any browned bits from the bottom. Place the seared chicken back into the pot, ensuring it is nestled among the onions. Cover the pot, reduce the heat to low, and let it simmer gently for 40-50 minutes, until the chicken is tender and fully cooked.

After simmering, taste the sauce and adjust the seasoning with additional salt, pepper, or a pinch of sugar if needed. Remove the bay leaves from the pot.

To serve, place the tender chicken and rich onions over a bed of fluffy rice or couscous, allowing the sauce to soak into the grains. Garnish with freshly chopped herbs.