Heat the oil in a large skillet. Season the pork loin generously on all sides with the seasoning mix.
Brown the pork loin in the hot oil, turning to brown all sides. The seasoning mix will form a light crust. Transfer the pork loin to the insert of the slow cooker.
Add the onion to the drippings in the skillet and cook, stirring, until very tender. Place the onions atop the roast in the slow cooker.
Pour the chicken stock and the sherry around the pork loin in the crock. Cover and cook until the pork is tender, about 4 hours on HIGH or 5 to 6 hours on LOW.
Remove the pork roast from the slow cooker along with the onions and place on a platter. Tent with foil.
Strain the drippings through a fine mesh sieve and measure out 2 cups, adding more chicken stock if needed.
In a large skillet, melt the butter. Saute the mushrooms until they release their liquids and become very tender. Sprinkle the flour in and stir to coat. Cook on low for 1 to 2 minutes. The flour and mushroom mixture will smell a little toasty. Slowly add the 2 cups drippings into the pan, stirring continuously until well incorporated. Simmer for several minutes until the gravy is thickened to your desired thickness. Serve the pork roast with the gravy immediately.
Notes
Wrap and store leftovers in the fridge up to 4 days.