Heat vegetable oil in a deep fry pan. Sauté garlic, onion, carrot, and celery for 2to 3 minutes. Add apples, breadcrumbs, and thyme. Stir to get everything well mixed.
Add Worcestershire sauce and apple juice.
Season with salt and ground pepper. Continue to cook until all water is absorbed, about 5 minutes. Turn off stove.
Preheat oven to 400°F (200°C).
Prepare Pork Sirloin Roast
Using a boning knife (or similar), insert the knife just under the silver skin and slice all the way through the side until all the silver skin is removed.
Cut open the sirloin roast lengthwise at 1/3 of the thickness from the bottom but do not cut through.
Then, cut the remaining 2/3rd into half from the fold outwards so that it lays flat with three times the original spread.
Sprinkle liberally with salt and pepper. Spoon prepared stuffing over the prepared spread-out pork sirloin.
Carefully fold 1/3 of the pork sirloin over the middle. Then, fold the other 1/3 over the top.
Using kitchen twine, tie the stuffed pork sirloin at the top end. Make a loop with the twine, then thread it around the meat and position it 1½inch (or closer if preferred)from the first piece of string. Hold on to the end of the first piece of string and pull tight to secure. Repeat this process all the way down the meat until you reach the end.
Sprinkle prepared roast all over with salt and pepper.
Place roast onto a wire rack in a deep pan. Brush melted over the top of the roast. Pour in 2 cups/480ml (or sufficient) water to cover the base of the pan with a ¼ inch water.
Transfer to the preheated 400°F (200°C) oven and roast for 15 minutes. Reduce oven temperature to 350°F(180°C) and continue roasting for another 1hour to 1 hour 15 minutes or until internal temperature of roast (not stuffing)is about 150˚F (65°C).
Remove and allow roast to restfor 10 minutes before removing string and slicing.