For the sauce: In a medium saucepan over medium heat, melt butter. Add onion and garlic and saute for 3 minutes, until softened. Add the rest of the ingredients; bring to a boil, then simmer until sauce is reduced to 2 cups, about 30 minutes.
Preheat grill pan. Season steak with salt and pepper and rub with oil. Grill steak about 3 minutes per side for medium-rare. Let rest 10 minutes and serve with sauce on side.