Pork Loin in Tamarind Sauce (Pernil de Cerdo en Salsa de Tamarindo)

1 cup tamarind pulp (from 10 shelled tamarind pods), or 1 cup frozen tamarind pulp, thawed
2 cups beef stock or canned broth
3 tablespoons sugar
1 (3-to-3-1/2-pound) boneless pork loin, tied crosswise at 1-inch intervals
2 tablespoons corn oil
1 teaspoon dried oregano
1/2 tablespoon salt
1/2 tablespoon black pepper

Combine the tamarind pulp, stock, and sugar in a nonreactive saucepan. Bring to a boil, reduce the heat, and simmer for 5 minutes. Strain the sauce through a colander; discard the solids. Set aside. Preheat the oven to 350 F. Rub the pork with oil, oregano, salt, and pepper. Transfer the meat to a roasting pan. Roast for 1-1/2 hours, or until a meat thermometer inserted reads 150 F. Transfer to a carving board and let stand for 10 minutes. To serve, warm the sauce over medium heat. Remove and discard the string from the pork. Carve the meat into 1/4-inch slices. Transfer a portion of the pork onto each dinner plate and spoon some sauce on top.