Seared Steak with Balsamic Red Onions

This absolutely easy and delicious meal has been a staple in our house since our friend Courtney served it at her home many years ago. She served it with deliciously creamy mashed potatoes, but I've come to enjoy it most served over brown rice.
Seared Steak with Balsamic Red Onions
Adapted from Real Simple Magazine

1 1/2 lbs sirloin steak, about 1 1/2 in thick, cut into 2 in cubes
2 tsp salt
2 tsp pepper
3 tbs olive oil
1 med red onion, cut into 1/2 in slices
1/3 cup balsamic vinegar

Season steak with salt and pepper on all sides.
Heat 1 tbs 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.