Start by chopping onion and sautee in a pan
Add salt and butter to onion
Pan fry until brown
Set aside the onion in fridge to cool down
Make sure onion is chilled before using
In a bowl, mix in ground beef, breadcrumbs, milk, egg, sauteed onion, salt, and pepper
Knead the beef mixture really well until all the ingredients are fully incorporated
Section the beef mixture into fourths (if you want to make smaller hamburger steaks, feel free to do so)
Scoop 1/4 of the beef mixture in one hand and shape into an oval
Toss the hamburger from one hand to the other hand to remove air pockets. This will make sure the hamburger cooks evenly without cracking
On medium heat, cook one side of the hamburger until brown (about 5 min)
Once it has a nice browning on one side of the hamburgers, flip the them over
Cover the pan and lower the heat to low
Once the hamburgers are fully cooked internally (about 10 minutes), remove them from the heat. I usually just crack one hamburger open to see that the inside is no longer red and raw. If you have a thermometer, then that will be more accurate.
Cook time will depend on size and thickness of the hamburger, so make sure to check. (USDA recommends internal temp for ground beef to reach 160F)