Rinse the chicken and trim and excess fat or connective tissue. Place in a medium bowl and add the mustard, 2 teaspoons of salt, and 1 teaspoon of pepper. Mix so that the ingredients evenly coat the chicken. Cover the bowl and place it in the refrigerator to marinate for a minimum of 10 minutes and up to 4 hours.
Wash the tomatoes well, then place in a small saucepan and cover with water. Bring to a boil and simmer for approximately 15 minutes or until the skins begin to break. Remove the tomatoes from from the water and transfer them to a blender.
Then, heat oil in a medium frying pan, add the sliced onion and whole garlic cloves. Saute them on medium heat, stirring frequently. When they begin to brown on the edges, turn off the heat and transfer them to the blender. Add the chipotle peppers including the adobo sauce, bay leaves and 1 teaspoon of salt to the blender and blend on high until a very smooth sauce forms.
In a large skillet or Dutch oven, heat the olive oil and butter over medium heat. Add the chicken and cook until it is nice and brown, about 5 minutes on each side.
Once the chicken is browned, pour the prepared chipotle sauce over the chicken in the skillet. Bring the sauce to a simmer, cover, and let the chicken cook for 20-30 minutes, or until it’s fully cooked and tender.
Once the chicken is fully cooked, taste is for salt and adjust as you like. Serve with your favorite sides like Mexican rice, beans, or warm tortillas.