Roast Pork Tenderloin with Apple-Onion Marmalade

3 tablespoons plus 1/4 cup olive oil
1 white onion, thinly sliced
1/3 cup balsamic vinegar
1/3 cup sherry vinegar
1 cup water
Salt and freshly ground pepper
2 pork tenderloins, about 3/4 lb each
2 fresh thyme sprigs
2 tablespoons unsalted butter
1 small green apple, peeled, cored and cut into 1/2-inch cubes
3 pitted prunes, thinly sliced
1 cup veal stock or chicken stock
2 tablespoons finely chopped fresh parsley
 
In a saute pan over medium heat, warm the 3 tablespoons olive oil. Add the onion and saute until golden brown, about 5 minutes. Add the balsamic vinegar, sherry vinegar, water and salt and pepper to taste and cook until the liquid has evaporated and the onions are very soft, about 45 minutes. Set aside.
Preheat an oven to 450 degrees F.
Rub salt and pepper to taste on all sides of the tenderloins. Place them in a roasting pan. Pour the 1/4 cup (2 fl oz/60 ml) olive oil over the top. Place 1 thyme sprig on each tenderloin.
Place the pan in the oven and roast the pork for 10 minutes. Turn the pork over and roast until firm and pale pink in the center when cut with a knife, about 10 minutes longer.
While the pork is cooking, in a large saute pan over medium heat, melt the butter. Add the apple and prunes and saute until slightly soft and caramelized, 3-5 minutes. Add the onion marmalade mixture to the pan and continue to saute until the flavors have blended, 2-3 minutes longer.
Add the stock to the pan and bring to a boil. Immediately remove from the heat and cover to keep warm.
When the pork is done, transfer it to a cutting board, cover with aluminum foil and let rest for 5 minutes. Then, using a sharp knife, cut the pork tenderloins into slices 1/2-inch (12 mm) thick. Arrange the pork slices on a warmed serving platter.
Spoon the warm marmalade mixture over the pork. Sprinkle with the parsley and serve immediately.