In a small bowl, combine spices and salt and pepper. Liberally season the steak with the rub and let rest 15 minutes on the counter.
Meanwhile, in a saute pan, heat 1 tablespoon oil and 1 tablespoon butter over low heat. Add onions, season and cook until soft and caramelized, about 20 minutes.
In a large saute pan, heat the remaining 1 tablespoon vegetable oil with 1 tablespoon butter over high heat. Place steaks in the pan and sear until nicely browned, about 3 minute per side for medium-rare. Remove pan to plate to rest 10 minutes.
Place the saute pan over high heat and deglaze with wine and vinegar until reduced by 1/2. Add onions to the pan with beef broth and reduce by half. Salt and pepper, to taste. Remove from heat and whisk in remaining 2 tablespoons of butter. Slice steak and fan to serve. Top with sauce.