In a food processor, blend the White Onion (2), Garlic (17 clove), Salt and Pepper (to taste), Fresh Thyme (2 sprig), Red Wine Vinegar (1/4 cup), Orange Juice (2 cup), Dried Oregano (1 tablespoon), Dried Rosemary (1 tablespoon), Bay Leaf (2), Vegetable Oil (1/2 cup) and Worcestershire Sauce (2 tablespoon) until getting a smooth paste.
Add the marinate to the Pork Shoulder (6 kilogram) and massage it into the meat, add Fresh Rosemary (2 sprig) cover and leave in the fridge until next day.
Preheat oven at 400 degrees F (200 degrees C).
Remove the pork from the fridge 1 hour before roasting, cover with aluminium foil.
Place it in the oven for approximately 4 hours.
Once cooked, remove the aluminium foil, increase the temperature to 450 degrees F (220 degrees C) and leave to roast until golden brown, approximately 1 hour.
Once the pork is cooked and golden brown, remove from the oven and wrap it in aluminium foil to rest for at least 20 minutes.
In the meantime place the roasting tray over the hob and add the Madeira Wine (1/2 cup), Ground Black Pepper (1/4 teaspoon), Worcestershire Sauce (1 teaspoon) and the All-Purpose Flour (1 teaspoon) if using. Mix everything with a whisk to dissolve the flour.
Leave to boil for approx 10 minutes.
After, pass the gravy through a sieve or fine colander to remove all large bits and finish with a smooth gravy.
Remove the pork from the foil, carve in thin slices and drizzle some of the gravy on top, serve immediately.