Preheat the oven to 400 degrees F. Allow the pork to stand at room temperature for 30 minutes.
Place the pork, fat side up, in a roasting pan just large enough to hold it comfortably. In a small bowl, whisk together the olive oil, 2 teaspoons of each mustard, the fennel seed, 2 teaspoons salt, and 1/2 teaspoon black pepper. Rub the mixture on top of the pork and roast for 1 to 1 1/4 hours, until the internal temperature reaches 140 degrees F. Remove from the oven, transfer to a cutting board, and cover tightly with aluminum foil for 20 minutes.
For the sauce, remove all but 1/4 cup of fat from the roasting pan. If there isn't 1/4 cup, add enough butter to the pan to make 1/4 cup total. Over medium heat, whisk the flour into the fat in the pan and cook for 1 minute. Add the wine and scrape up all the brown bits from the bottom of the pan. Add the chicken stock, the remaining 2 teaspoons of each mustard, the green peppercorns, 2 teaspoons salt, and 1/2 teaspoon black pepper. Bring to a boil, then lower the heat to a simmer for 5 to 10 minutes, until the sauce is reduced and slightly thickened.
Remove the strings from the roast pork, slice between the bones, and serve warm with the hot sauce.
Homemade Chicken Stock:
3 (5-pound) chickens
3 large onions, unpeeled and quartered
6 carrots, unpeeled and halved
4 celery stalks with leaves, cut in thirds
4 parsnips, unpeeled and cut in half (optional)
20 sprigs fresh flat-leaf parsley
15 sprigs fresh thyme
20 sprigs fresh dill
1 head garlic, unpeeled and cut in 1/2 crosswise
2 tablespoons kosher salt
2 teaspoons whole black peppercorns
Place the chickens, onions, carrots, celery, parsnips, parsley, thyme, dill, garlic, salt, and peppercorns in a 16 to 20-quart stockpot with 7 quarts of water and bring to a boil. Skim the surface as needed. Simmer uncovered for 4 hours. Strain the entire contents of the pot through a colander, discarding the chicken and vegetables, and chill. Discard the hardened fat, and then pack the broth in quart containers.