Pan-Seared Rib-Eye Steak with Balsamic Onion & Tomato Salsa

Heat the oil in a 12-inch skillet over medium-high heat until hot; add the onion and sugar. Cook the onion, stirring occasionally, until it begins to soften but not brown, 4 to 5 min. Add the vinegar and cook until the onions are soft, 2 to 3 min. Add the tomatoes and cook, stirring, until they just begin to soften, 1 to 2 min. Remove the skillet from the heat, stir in the basil, and season the mixture to taste with salt and pepper. Transfer the mixture to a bowl and keep warm. Season the steaks generously on both sides with salt and pepper. Wipe out the skillet and melt the butter in the skillet over high heat. Cook two of the steaks until nicely browned on both sides and cooked to your liking, about 3 min. per side for medium rare. Transfer the steaks to plates or a platter and keep warm while you cook the remaining two steaks. Serve the steaks topped with the onion and tomato salsa.