Heat vegetable oil over high heat in an oven-proof, heavy-weight sauce pan. Season pork with salt and pepper and brown on all sides. Add butter, cover, and cook in oven — about 35 to 45 minutes.
When cooked, place meat on a plate and cover with foil. Let rest for 20 minutes. Drain fat from sauce pan, add water, bring to a boil over high heat, and cook until reduced by two-thirds.
For sauce: heat butter in a small sauce pan over medium heat. Sweat shallots until soft. Add white wine and reduce by half. Incorporate the mustard and cook a couple of minutes. Strain the reduced pan drippings through a chinois into the mustardsauce and mix. Adjust seasonings.
Slice the pork and arrange on serving plates. Drizzle sauce over meat slices.