Whisk first 4 ingredients in a large 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 a large saucepan.

Boil until reduced to 1 cup, about 20 minutes.

Add stock and boil until reduced to 1 ¼ cups, about 20-25 minutes longer.

Set aside.

(can be prepared 4 hours ahead. Cover and refrigerate. Bring steaks to room temperature before continuing) Divide oil between 2 large heavy skillets and place over high heat.

Season steaks.

Add 3 steaks to each skillet and brown on both sides.

Reduce heat to medium-high and cook to desired doneness, about 4 minutes per side for medium-rare.

Transfer steaks to warm plates.

Tent with foil to keep warm.

Add half of sauce to each skillet and bring to boil, scraping up any brown bits.

Transfer contents of 1 skillet into the other and bring to slow simmer.

Add butter, whisking in one piece until melted before adding the next.

Spoon over steaks and serve.