Divide meat into 4 equal portions. Form each portion into a ball and press down on a plate to form a patty. Continue pressing out until as thin as possible while still maintaining a burger shape (see pics in blog post).
Season one side with salt and pepper (a little less than 1/4 a tsp of kosher salt per patty; pepper to taste).
Preheat a large, nonstick skillet over high heat. Spritz seasoned side of burgers with cooking spray and place them in the skillet, seasoned side down, so that the burgers aren't touching. Let cook for about 3-4 minutes before flipping.
While the first side of the burgers cook, spread mustard on the raw side of the burgers, about 1 1/2 teaspoons for each burger (see pics in blog post). Spritz mustard side with cooking spray.
Flip burgers and top cooked side with cheese. Cook mustard side of the burgers for another 2-3 minutes, or until beef is done to your liking. Let rest for a few minutes.