Preheat oven to 300°F Butterfly pork shoulder to create a 2" thick rectangle of meat. Cover with a layer of plastic wrap (heavy duty is best) and pound with a mallet until it is about 1" thick. Remove plastic wrap.
Season generously with salt and pepper. Combine fennel fronds, garlic, rosemary, ground fennel seeds and red pepper flakes in a small bowl. Remove and reserve 1/3 of this mixture; sprinkle the rest evenly over the surface of the meat.
Roll pork tightly. Tie with butcher's twine every 2-3". Season the outside with salt, pepper, and the reserved spice mixture.
Scatter onion, carrots and celery into a 9x13 baking pan. Place pork on top of vegetables in center of pan. Pour chicken broth around pork. Roast in preheated oven about 75 minutes, until thermometer inserted into center of meat registers 150°F Remove roast to a plate, tent with foil and let stand 10-15 minutes before removing twine and slicing. (Discard vegetables and broth.).