Butterfly the pork roast (slice in half lengthwise to within about 1-1/2 inches of the uncut edge). Open the roast like a book, and slice a bit more in the center if needed so it lies flat all the way across. Lightly score the meat in a cross-hatch pattern to maximize surface area for seasoning.
Combine olive oil, salt, pepper, garlic, fennel seed, and fresh parsley in a small bowl. Mix well. Spread half the seasoning goo on the outside of the roast, then flip and spread the remaining goo on the inside (the scored side). Spread the chopped fresh fennel in the right half of the inside (scored) side of the roast, spreading out to within a half inch of the edges.
Fold the left side over the right (close the book!) and carefully pick up the roast and set it in an oiled 3-quart slow cooker insert. If you like, you can tie the roast with cotton kitchen twine (I rely on the smallish slow cooker to keep the roast together, but if you want absolute certainty or are using a larger pan, by all means tie it up!).
Let the roast marinate overnight in the fridge (up to two days). When ready to cook, splash about an inch of good white wine in the bottom of the slow cooker. Cook for 6-7 hours on low. (Alternately, you could roast it at 325 for probably 2 to 2-1/2 hours -- I don't have an exact time since I use the slow cooker, but since you managed to find this recipe, I'm confident you can look up a cooking time chart and figure that out, too!).
Once cooked, carefully transfer the meat to a large plate and shred. Set aside while you make the gravy.
To make the gravy, pour the drippings out of the slow cooker insert and into a wide, shallow pan (a 10- or 12-inch skillet works marvelously for this). Heat skillet over medium high heat till liquid is simmering. In a small bowl, combine the flour with a few spoonfulls of the heated drippings, whisking to remove any lumps. The mixture should resemble runny paste. Once mixture is lump-free, whisk into the rest of the drippings and cook at a simmer, stirring constantly, for 4-5 minutes, till thickened. Add wine (water works, too) to achieve the desired consistency (I add about 1/4 cup, but the amount will vary depending on how much drippings your roast gave you). You will want to taste-test to make sure the gravy doesn't taste "floury" (if it does, cook it a bit longer).
Serve the hot meat in open-faced sandwiches on good bread. Serve the leftovers hot or cold on buns.