Rub pork all over with salt. Coarsely grind seeds and peppercorns in a spice grinder or with a mortar and pestle; rub all over pork. Bring pork to room temperature, about 30 minutes. Meanwhile, preheat oven to 425 degrees.
Rub pork with oil to coat generously. Roast until sizzling, about 20 minutes.
Reduce heat to 375 degrees. Baste pork, and roast until center reaches 145 degrees, about 30 minutes more. Let pork rest 15 minutes before slicing. Meanwhile, toss fennel and chiles with just enough oil to coat; season with salt and pepper. Spread in a single layer on a rimmed baking sheet; roast until tender and lightly browned on bottom, 25 to 30 minutes. Add olives, and roast until hot, about 5 minutes. Toss together; serve with pork.