For Pork Roast: Position rack in lowest third of oven and preheat to 475°F Add 1 tablespoon butter to a heavy large skillet over medium-high heat and roast and cook until brown, about 4 minutes per side. Remove from skillet. Cool 10 minutes. Transfer to roasting pan.
Combine remaining 1/4 cup butter with mustard seeds, flour, sugar, mustard, thyme, and peppercorns in bowl. Spread paste oven top and sides of roast. Roast 30 minutes. Reduce heat to 325 degrees.
Continue cooking about 1 hour 20 minutes. Transfer roast to cutting board and tent with foil. Transfer 2 tablespoons drippings in pan to heavy small saucepan; discard remaining drippings.
For Cider Gravy: Heat roasting pan over medium-low heat. Add cider and boil until liquid is reduced to 3/4 cup scraping up any browned bits, about 8 minutes. Stir in apple jack, boil 1 minute.
Heat drippings in saucepan over medium-high heat. Add flour and stir until golden brown, about 2 minutes. Whisk in cider mixture and stock. Simmer until thickened, stirring occasionally, about 2 minutes. Remove from heat. Mix in vinegar and mustard. Season with salt and pepper. Carve roast and serve with gravy.