Cut the roast in half lengthwise and open it like a book. Combine the salt, pepper, garlic, parsley, fennel seeds and olive oil in a small bowl. Rub the seasoning mixture all over the pork, pressing the fennel seeds and garlic into the meat. Spread the chopped fennel bulb over the meat, the fold the meat back together and tie it with a string.
Place the meat in a roasting pan, cover and bake for 2 1/2 to 3 1/2 hours. Toward the last 40-50 minutes of cooking, scatter the potatoes, carrots, parsnips, and onions in the roasting pan around the meat, and continue to cook until pork is cooked through and falls apart when touched with a fork. Stir the vegetables now and then so they brown evenly.
Allow meat to sit for 5 minutes on a cutting board. To serve, pull the pork apart (instead of slicing), and serve with the roasted vegetables. Garnish with feathery fennel tops.