Heat the oven to 400° F. Heat the pan over medium heat and add the two tablespoons of oil. Add the shallots and a little bit of salt, and seat out the shallots until they are soft, about 5 minutes.
Pat the chicken with a paper towel and season with salt and pepper.
Add the potatoes and tomatoes to the pan to the shallots and cook for another minute. Add the mustard and stir it in. Add the sherry or wine, vinegar, and stock and bring to a simmer. Add the chicken to the pan skin side up. Place the pan in the oven and roast without a lid for about 40-50 minutes.
When you take out of the oven, pull the chicken to a plate to rest. Pour all the juices from the pan into a bowl or la fat separator. Let the cooking liquid sit for a few minutes until the fat rises to the surface. Use a spoon or ladle to remove fat (our pour if using the separator). Taste the sauce. Adjust with salt or pepper. Return the sauce, shallots, potatoes, tomatoes and chicken to the pan. Bring to a gentle simmer for a couple minutes. Serve chicken, potatoes and tomatoes with plenty of sauce. And absolutely positively serve with crusty bread.