1 1/2 lb. sirloin steak, about 1 1/2 in thick, cut into 2 in cubes
2 tsp. salt
2 tsp. pepper
3 T. olive oil
1 med red onion, cut into 1/2 in slices
1/3 C. balsamic vinegar

Season steak with salt and pepper on all sides. Heat 1 T. oil in large non-stick pan over high heat. Add steak in batches and cook 4-5 minutes for med-rare. Remove steak from pan and cover to keep warm. Reduce heat to med-high and add onion, tossing until crisp tender and lightly browned, about 5 minutes. Add balsamic vinegar and cook 2 minutes more, or until reduced to a syrupy consistency. Add the steak back into the pan and toss it with the onions and vinegar.