Heat the oil in a large heavy sauté pan with a tight-fitting lid over medium heat. Season the chicken legs well with salt and pepper and when the oil is almost smoking, add them to the pan. Brown them well on all sides; this should take about 5 minutes. Transfer the chicken to a clean plate and set aside.
Add the onion and peppers to the pan and cook until beginning to soften, about 3 minutes. Add the garlic and cook for another minute. Stir in the mustard and cook for another minute. Stir in the tomatoes and a couple large pinches of salt, then add the chicken back to the pan, submerging it in the sauce.
Bring the sauce to a boil and then lower the heat so that it's just simmering. Cover tightly and cook until the chicken is very tender, 30 to 40 minutes. Taste for seasoning and add more salt and pepper if you like. Serve over rice or couscous.