In a bowl, mix all the ingredients for the special sauce then place in the fridge until ready to use.
Slice your onions into 1” thick onion slices as a whole to use as your “buns”.
Heat your grill to high heat (around 450 degrees F) for direct cooking.
Grab a handful of ground beef (around 1/4 lbs… ish) and gently roll it into a ball. Repeat this for 6-8 balls or until you’re out of meat.
Add a cast iron skillet or plancha over the flames to heat till it's slightly smoking (about 450 on the skillet).
Add a burger ball to the skillet. Place a piece of parchment over one ball and then firmly press with the spatula until the burger is “smashed” on the skillet.
Season the patty with salt, pepper, and garlic. Let cook for another 1-1.5 minutes. Before flipping, drizzle some yellow mustard on the unflipped patty.
Carefully flip the burger patty over and then season the new side. Top with a slice of cheese and let melt for 1-2 minutes.
Once melted, pull off and rest. Repeat for the rest of your patties.
Add your sliced onions to cook for 2 minutes per side. Once browned, top with a slice of cheese and let melt.
Build your burger with onion slice, burger sauce, smash burgers, more sauce and another onion slice. Serve and enjoy!