Rinse pork and pat dry.
In a food processor (a mini model works best), blender, or mortar with pestle, coarsely grind or crush fennel seeds with sage, peppercorns, and 1 1/2 teaspoons salt. Rub all the fennel-seed mixture evenly over pork, tucking some into crevice where roasts join.
Set meat on a rack in a 9- to 10-inch by 13- to 15-inch rimmed pan. Bake in a 375° regular or convection oven until a thermometer inserted in center of roast reaches 150°, about 2 1/4 hours.
Transfer roast to a platter; let rest in a warm place for at least 10 and up to 30 minutes.
Meanwhile, skim and discard fat from pan drippings. Add chicken broth to pan; scrape bottom to release browned bits. Set pan over high heat and stir until boiling. Pour any juices accumulated on platter into pan. Pour mixture through a fine strainer into a small pitcher.
Slice meat and serve with pan juices, salt to add to taste, and fennel and Belgian endive casserole.