Adjust the top rack in your oven so it’s about 4 inches from the broiler and heat broiler on high. Transfer meat to a foil-lined heavy-duty baking sheet or broiler pan. Place onion rounds around the meat on the baking tray wherever they will fit.
Broil for 4 to 6 minutes per side (flip both the meat and the onion rounds), until an instant-read thermometer inserted into the thickest part of the steak reads 135°F for medium-rare, 145°F for medium.
If onions have not yet reached your desired level of doneness, you can transfer the meat to a cutting board and return onions to the oven. Once onions are done, slice the rounds in half, separate the slices, and toss them in the pan juices. Allow the meat to rest for 5 minutes before slicing across the grain into strips.