Preheat oven to 400°; place the fennel seeds on a cutting board and break them by cutting once or twice with a large heavy knife (if you have a mortar and pestle, give them a couple grinds to lightly crush and release their aroma).
Combine the fennel, garlic, salt, and pepper; stir to combine.
Rub the fennel seed mixture over the entire surface of pork roast.
Place the roast in a 13x9 inch baking dish; roast for 30 minutes.
Remove pan from oven; using large tongs or two spatulas, carefully turn the roast over.
Return to the oven; continue roasting about 30 minutes until well browned.
Remove roast from oven; let stand about 10 minutes.
Carefully remove string and cut meat into thin (1/8 inch) slices; arrange the overlapping slices on a platter; spoon any pan juices over the meat and serve.