Rinse pork loin to remove residue and pat dry with paper towels. Rub with the mesquite seasoning. Then combine the ground cloves, cinnamon, allspice and sugar and rub onto pork as well. Let sit refrigerated for at least 2 hours.
Place dried shiitakes into a small bowl and cover with hot water for about 20 to 30 minutes to re-hydrate.
Preheat oven to 350 degrees F. Heat oil over medium-high heat in a large saute pan with oven safe handles. Sear pork loin on all sides and roast in oven until fork-tender, about 1 hour. Remove from oven, transfer pork loin to a cutting board, and let rest.
In the last 30 minutes of the cooking time for the pork loin make the sauce. Melt the butter over medium heat in a large saute pan with a cover. Add the onion and saute until it becomes translucent. Add the apples and pour the sherry over, cover and let cook until the apples begin to soften, about 10 to 15 minutes. Strain the mushrooms through paper towels and reserve the soaking liquid in a small bowl. Whisk the cornstarch into the soaking liquid along with mustard powder, salt and pepper. Add the soaking liquid mixture to the apples and stir to coat. Let cook for a few minutes to let flavors integrate, then remove from heat, adjust salt and pepper as needed, and keep covered in a warm place.
Slice pork loin down and transfer to a serving dish. Spoon sauce and apples over the meat and garnish with parsley sprigs.