In a mixing bowl, combine the 90% lean ground beef with Worcestershire sauce, kosher salt, and black pepper. Mix gently until just combined; be careful not to overwork the meat to keep it tender.
Heat your cast iron skillet over medium-high heat. If you're using lean beef, add the canola oil to the skillet.
Divide the beef mixture into three equal portions. Roll each portion into a loose ball and place them in the hot skillet. Press down firmly on each ball to flatten it into a patty about ½ inch thick. Let it cook undisturbed for about 2-3 minutes.
Once the edges of the patties start to brown and the juices begin to rise on top, flip the burgers. Immediately place a slice of cheese on each patty. Cook for another 1-2 minutes until the cheese is melted and the burgers are cooked through.
Add the whole grain or brioche buns, cut side down, to the skillet for about a minute or until golden and crispy.
Remove the patties from the skillet and place them on the toasted buns. Top with your favorite burger toppings. Serve immediately and enjoy your delicious Smash Burger!
