Place the chicken legs, the onions and the pepper in a grease-free mixing bowl and add the mustard, the lemon juice and the vegetable oil. Marinate the chicken in the bowl with the remainder of the ingredients and place the mixture in the refrigerator for at least 4 hours (the longer the better).
Remove the chicken legs from the mixing bowl, pat them dry and bake them on both sides until golden brown.
Heat the olive oil in a large pan over medium heat and fry the onions (from the marinade) at least 8-10 minutes.
Add the rest of the marinade and the chicken legs to the pan and allow to simmer for 30 to 40 minutes on a low heat until fully cooked and tender.
Add salt and pepper according to taste and serve with rice or couscous.