Form beef into one evenly shaped patty. Season each side with salt and pepper.
Fill a pot halfway with water. Place steamer basket on top and place on a burner. Set to high and bring water to a boil.
Place cheese cubes in a ramekin cup safe to use. Carefully place cheese cup and burger into the steamer basket. Cover with the lid. Cook until medium rare at 130 F, about 10 minutes. Use thermometer to check for doneness.
Carefully remove the cheese cup and burger with tongs. Allow burger to rest for a minute.
Place burger on bottom bun and use a knife to scrape out melted cheese onto burger. Top with desired toppings- onion, lettuce, tomato, mayo and mustard.