Step 1: Prep Ahead (Do this the day before or morning of!)

1. Caramelize the Onions: In a large skillet over medium-low heat, melt the butter. Add diced onions and a pinch of salt. Cook for 20-25 minutes, stirring occasionally, until they are golden brown and jammy. Set aside to cool, then refrigerate until needed. These can be gently reheated or served room temp.
2. Mix the Spread: Whisk all of the spread ingredients in a small bowl until combined. Cover and refrigerate.
3. Season the Beef: In a bowl, gently mix the salt, garlic powder, onion powder, and Worcestershire sauce into the ground chuck. Form into 1.5–2 ounce balls (roughly golf-ball sized). Place them on a plate, cover, and refrigerate.
4. Shred Lettuce: Shred your iceberg lettuce and keep it in an airtight container in the fridge so it’s super crisp.

Step 2: Game Day! Sear & Assemble

You’ll want to serve these right after searing them and they come together quick, so be sure to make them near the time people have already arrived or when everyone wants them! You can also keep them on a sheet tray in the oven on Warm as you make them all. 

1. Heat the Griddle: Get a cast-iron skillet or flat-top griddle hot over medium-high heat. You don’t need oil if your pan is well-seasoned or non-stick. Otherwise, add 1/2 Tbsp oil. 
2. The Smash: Add 1 Tbsp of mustard on the bottom of the cast-iron (like In n Out does!). Place a beef ball on top of the mustard. Immediately place a tortilla directly on top. Using a heavy spatula or burger press, smash down until the meat is paper-thin and extends to the edges of the tortilla.
3. The Flip: Let it sear for 2-3 minutes until the meat edges are dark brown and crispy. Flip the entire taco so the mustard-seared beef is facing up, and the tortilla is now on the griddle.
4. Melt & Assemble: Immediately place a slice of American cheese on the meat. Let the cheese slightly melt while the other side of the tortilla heats up. Remove after about 30 seconds. 

5. Finish: Fold in half to make a taco shape and to continue to let the cheese melt. At this point you can serve it for someone to add their toppings (grilled onion, shredded lettuce and secret sauce) or place on a sheet tray and keep it in the oven on Warm. Repeat with remaining beef and tortillas!

Note: The first few tacos might take 3-4 minutes to get that nice sear on the meat but as you continue going the pan will become scorching hot. Be sure to turn the heat down (or even off) to let it cool for a minute so as not to burn your 7th or 8th smash taco!