Whisk first 4 ingredients in lg bowl. Divide steaks between 2 glass baking dishes. Pour marinade over. Cover and refrigerate overnight.

Remove steaks from marinade and pat dry. Transfer marinade to heavy lg saucepan. Boil until reduced to 1 cup, about 20 minutes. Add stock and boil until reduced to 1 1/4 cups, about 20 minutes. (can be prepared 4 hours ahead. Cover and refrigerate steaks and sauce separately. Bring steaks to room temp before continuing)

Divide oil between 2 heavy lg skillets and place over high heat. Season steaks with salt and pepper. Add 3 steaks to each skillet and brown on both sides. Reduce heat to med high and cook to desired doneness, about 4 min. per side for medium-rare. Tent with foil to keep warm. Add half of sauce to each skillet and bring to boil, scraping up any browned bits. Transfer contents of both skillets to one skillet and bring to simmer. Add butter and whisk until melted. Spoon sauce over steaks and serve.