Preheat oven to 450 degrees.
Season tenderloin with salt and pepper.
Over medium-high heat, heat 2 tablespoons of olive oil in a large non-stick oven-proof skillet. Add pork and sear until all sides are brown, about 5 minutes, turning occasionally. Transfer to a plate to cool slightly.
Spread mustard over top and sides of pork; press fennel seeds into the mustard. Add remaining 1 TBS oil to skillet. Add fennel, onion and apple slices to skillet spreading evenly. Sprinkle with salt and pepper. Saute over medium heat until golden, about 5 minutes.
Place the pork on top of the fennel, onion and apple mixture. Transfer skillet to oven and roast until fennel, onion and apple mixture is soft and brown, using a meat thermometer, checking that the pork reaches 150 degrees. This will take approximately 15 minutes. When done, transfer pork to a platter and
tent with foil. Allow to stand for 5 minutes.
Meanwhile, in a skillet, pour apple cider over the fennel, onion and apple mixture. Add cinnamon stick. Stir mixture over high heat until slightly reduced, about 2 minutes. Cut pork on diagonal into half-inch thick slices. Spoon fennel, onion and apple mixture onto plates. Top with pork and serve.