3 pound boneless center rib pork roast
1 cup fresh fennel bulb and leaves, diced
4 sprigs fresh rosemary
2 garlic cloves
3 tablespoons extra-virgin olive oil
½ cup carrot, diced
½ cup celery , diced
½ cup onion, roughly chopped
2½ cups chicken stock, or canned low-sodium chicken broth
Heat the oven to 450 F. Place the fennel, garlic and leaves from 2 of the rosemary sprigs on a cutting board and chop them together until minced. Transfer the mixture to a mortar and add 1 tablespoon of the olive oil. Grind the mixture to a paste with a pestle. Season with salt and pepper. (Alternatively, the mixture can be ground fine in a blender, stopping often to scrape down the sides of the jar.)
Spread half the fennel mixture along the cut in the roast. Fold the flap over the opening and tie the roast securely with kitchen twine at 2-inch intervals. Thread the 2 remaining sprigs of rosemary through the ties on either side of the roast. Make holes in the top and sides of the roast with a paring knife or thick skewer and force the remaining vegetable mixture into the holes. Season the outside of the roast generously with salt and pepper, then rub with the remaining 2 tablespoons olive oil. Set the roast in an 18 x 14-inch roasting pan.
Roast 15 minutes. Lower the oven temperature to 400 F. Tilt the roasting pan and spoon off excess fat from the roast. Scatter the carrot, celery and onion around the roast. Roast 15 minutes.
Pour the stock into the pan and continue cooking, basting the roast occasionally with the pan juices until a meat thermometer inserted into the thickest part of the roast registers 155 F, 40 to 50 minutes.
Remove the roast to a platter. Pass the vegetables and pan juices through a food mill fitted with the fine disc into a small saucepan. (Alternatively, strain the liquid through a sieve, pressing on the vegetables to extract as much liquid as possible and force some of the vegetables through the sieve.) Skim all fat from the surface of the sauce. The sauce should be thick enough to lightly coat a spoon. If not, transfer the sauce to a small saucepan and bring to a simmer. Simmer until thick enough to lightly coat a spoon. Season the sauce with salt and pepper, if needed. Cut the meat into 1/4-inch slices and serve with the sauce.