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Thinly slice the onions. Heat a large skillet over medium-low heat and add the butter. Once melted, add in the onions with a good pinch of salt. Stir to combine everything. Cook, stirring often, for 20 to 30 minutes, until the onions caramelize. I love a classic, slowly caramelized onion, but if you’d like to speed up the process, you can add a pinch of sugar while cooking. You can also make these ahead of time and store them sealed in the fridge, submerged in a bit of olive oil.
Divide the beef into 4 equal parts. I like to form mine into balls so they smash into a more circular burger.
Heat a large cast iron skillet or your grill griddle over medium-high heat. You want it to be hot, so heat it for at least 5 to 10 minutes.
Once it's hot and your burgers are ready to go, quickly brush the skillet with oil and place half of the butter on the skillet. It will smoke, so work quickly! Place one of the balls of beef on the skillet and smash it down with a burger press, spatula, mason jar, etc - once or twice. You want it to be about 1/2 inch in thickness (or a little less) and want to try and smash it only 2 or 3 times. Season it with salt and pepper and repeat with the rest of the beef. Cook the burgers for 2 to 3 minutes and then flip, season with salt on the other side and cook for 2 minutes more. Add the cheese and let it melt for just 60 seconds or so.
Remove the burgers from the heat. Spread the aioli on the buns and top with a piece of lettuce. Add the burger on, along with the caramelized onions. Serve immediately!
Whisk together the ingredients until combined. You can make this ahead of time and store it in the fridge.