Instructions:
- Mix sauce ingredients together. Set aside.
- Divide beef into 4 equal balls. Do not season yet.
- Heat a cast iron pan or griddle over very high heat until smoking.
- Place a beef ball on the pan and immediately smash flat with a spatula. Season with salt and pepper.
- Cook for 2 minutes until edges are crispy and brown. Flip, add cheddar and cook 1 more minute.
- Toast brioche buns cut-side down in the same pan.
- Assemble: sauce on both buns → lettuce → patty with cheese → onion → pickles → tomato → top bun. Serve immediately.
