Position a rack in the center of the oven, and heat oven to 450 F.
In a small bowl, mix the maple syrup, mustard, thyme, and pepper.
Drain the pork and pat dry with paper towels.
Brush the pork all over with the mustard mixture.
In a medium bowl, toss the fennel and apple with the oil, salt, and a good pinch of black pepper.
Scatter the mixture in the bottom of a large roasting pan (large enough to hold the pork with an inch or two of space around the edges).
Put the pork, fat side up, on top of the fennel and apples.
Roast the pork until the crust just starts to brown, about 20 minutes.
Reduce the heat to 350 F and continue cooking until an instant-read thermometer inserted into the thickest part of the loin reads 145 F., about 40 to 50 minutes more.
Remove from oven and let rest for 10 minutes.
Thinly slice the pork.
Serve, topped with the fennel, apple, and juices.