Heat cast iron skillet or heavy-bottom pan over medium-high heat for 3-4 minutes. Add oil and swirl to coat evenly. The pan should be hot enough that a drop of water sizzles immediately (about 375°F/190°C).
SECRET #1: Place beef balls in pan with space between each. Immediately smash down firmly with spatula or burger press for 10-15 seconds, creating thin patties about 1/4-inch thick. The thinner, the crispier the edges will become.
SECRET #2: Season the raw side generously with salt and pepper while the bottom sears. Do not move or flip for 2-3 minutes until deep golden crust forms and edges are crispy.
SECRET #3: Flip burgers only once when you can easily slide spatula underneath without resistance. Cook for additional 1-2 minutes until internal temperature reaches 160°F (71°C).
SECRET #4: Add cheese slices during the last 30 seconds of cooking and cover pan briefly to melt. Remove patties and let rest for 1 minute.