The day before, slice the top of the meat diagonally in a diamond pattern. Rub the garlic, oil and fennel seeds on top of the pork. Place orange slices from one orange on top.
Wrap the pork in plastic and place in the refrigerator overnight.
Preheat the oven to 325 degrees. Remove the plastic wrap. Add the onion slices and remaining orange slices to the bottom of a dutch oven. Place the unwrapped pork on top.
Cover and bake the roast for 3 hours. Remove the cover and roast for 30 minutes longer.