Boil wine, Port and broth in heavy Dutch oven until reduced to 6 cups, about

40 minutes.

Meanwhile, melt 4 tbs butter in large skillet over medium-high heat. Add onions and saute until tender, about 15 minutes. Mix in thyme and saute until onions are deep brown, about 10 minutes longer. Transfer onions to bowl. Melt 2 tbs butter in same skillet. Add mushrooms and saute until tender, about 15 minutes. Return onions to skillet. Add flour and stir 3 minutes. Stir mushroom mixture into wine mixture. Simmer over medium heat until thickened and reduced to 6 cups, stirring occasionally, about 1 hour. Season to taste with salt and pepper. (Can be prepared 1 day ahead. Cover and refrigerate.)

Preheat oven to 400°. Rub tenderloins with olive oil. Season with salt and pepper. Place in roasting pan. Roast until thermometer inserted into thickest part of meat registers 125°F, about 40 minutes for rare. Remove from oven and let stand 10 minutes.

Rewarm sauce over low heat. Mix in any juices from roasting pan. Slice tenderloins crosswise into 1/2² thick slices. Overlap beef slices on platter. Spoon some sauce over. Serve, passing remaining sauce separately.