In a frying pan heat olive oil over medium heat and sauté the chopped onion until it becomes translucent.
In a large bowl, combine the ground beef, sautéed onion, Worcestershire sauce, salt, and pepper.
Form the beef into 6 fairly thin patties. (About 5 inches in diameter) They will shrink and thicken as they cook.
Grill each patty on medium-high heat, about 5 minutes per side.
Assemble your burger according to your liking. I like to spread a little mayonnaise and Dijon mustard on each bun, then stack lettuce, onions, tomatoes, pickles, patty, sautéed mushrooms, and Havarti cheese.
Notes
Flatten the hamburger patties- Because the meat shrivels as it cooks, roll the meat out thin and large, at least 5 inches in diameter. This will avoid burgers that look like meatballs!
Cook them over high heat- In order to get a nice outer crust, you'll need to cook them over high heat so the outside can caramelize a little.
Check the temp with a thermometer- Depending on how well done you like your burger, check if they're done with a meat thermometer. Medium well=150-155°F.
Let them rest- Before you bite into one of these juicy hamburgers, let them rest a few minutes while you get the condiments together. This allows the juices to reabsorb into the meat for juicier results.