Score the top of the pork loin, with a knife, creating 1/8-inch deep X's. In a bowl combine the rosemary, parsley, thyme, grapeseed oil and salt and pepper, to taste. Coat the pork loin with the herb rub and refrigerate for 20 to 30 minutes.
Preheat the oven to 400 degrees F.
Put the pork on roasting rack, in a roasting pan. Roast until the pork is cooked through, about 25 to 30 minutes. Remove the pork from the oven to a cutting board and let it rest until ready to serve.
In a medium saucepan over high heat, add the beef stock, onion, herbs, wine, sugar and salt and pepper, to taste. Cook until the stock has reduced by 60 percent in volume. Remove from the heat and pour through a strainer, then return it to the pan over medium heat. Add the mango and continue to cook for 10 to 15 minutes, allowing the flavors to blend. Remove from the heat, whisk in the butter and keep warm. Slice the loin, arrange it on a serving platter and serve with the mango sauce.