Heat oven to 400° F.
Using the bottom of a heavy pan, crush the fennel seeds.
In a small bowl, mix the seeds, the garlic, 2 tablespoons of the oil, 1 teaspoon salt, and 1/4 teaspoon pepper. Rub the mixture over the pork, then place the pork in a large roasting pan.
In a bowl, mix the onions, potatoes, pears, 1 teaspoon salt, 1/4 teaspoon pepper, and the remaining oil. Scatter around the pork and roast until cooked through, about 70 minutes (internal temperature 160° F). Transfer the pork to a cutting board and let rest at least 5 minutes before slicing. Serve with the roasted produce.