With your non-dominant hand, hold the pork loin, firmly pressing the sides. Use a long, thin and very sharp knife to make a horizontal slit through the center of the pork all the way through to the other side, gently slicing about half an inch in either direction. Remove the knife and make the same cut, in the same area -- but perpendicular to the one you've just made -- so that it looks like you've cut a "plus sign" in the center of the pork roast. Place the roast in a plastic storage bag and pour the Stubb's Pork Marinade over. Seal the bag, releasing any excess air and work the marinade all around and inside the pork with your hands. Place the bag in a bowl (to catch any spills or drips) and refrigerate overnight.
Remove the crusts from the bread and cut into 1/2" dice. Place the bread in a bowl on the counter and let sit overnight uncovered to dry out even more and get stale. (You'll need about 5 cups of bread cubes).
The next day, preheat the oven to 325°.
In a large skillet, heat the olive oil over medium high heat. Add the onions, celery and apple and saute for 3-5 minutes until tender. Add the Stubb's Pork Seasoning, salt and pepper and stir to combine. Add the cherries and dried bread. Pour the broth over the bread and mix well, but don't break apart the bread. If it's a little dry, add a few more tablespoons of broth. Set aside to cool for a few minutes.
Remove the pork from the marinade and pat dry with paper towels. Use your clean hands to stuff the pork loin with the stuffing, pressing lightly with your fingers until you reach the center of the pork. Flip the pork over and add stuffing from the other side, until the pork loin is completely filled. Sprinkle the pork with Pork Spice Rub, salt and pepper.
In a small bowl, toss the onions and apple with olive oil and spread out in the bottom of a roasting pan. Place the pork loin on top and roast for one hour.
There will be leftover stuffing, so spray a baking dish that with vegetable spray and transfer the remaining stuffing to the dish. Dot with a tablespoon of butter, cut into small pieces. 20 minutes before the pork is finished, add the stuffing to the oven and continue cooking.
For the Gravy
In a small saucepan add the chicken broth and apple cider and bring to a boil. Cook until the liquid has reduced to about 1 1/3 cups. In another saucepan, over medium high heat, melt the butter and add the flour, whisking to combine. Cook for one minute and stir in the thyme and Calvados, cook for another minute and then stir in the reduced stock-cider. Whisk while cooking until the liquid comes to a boil and thickens slightly. Stir in the cream, salt and pepper to taste. Remove from heat and transfer to a gravy boat.
Slice the pork into 1/3" thick slices with the gravy and stuffing.