Pickled Red Onions:
1 cup apple cider vinegar
1½ tablespoons sugar
1 tablespoon salt
1 red onion, halved and sliced into half moons
1 egg yolk
2 teaspoons Dijon mustard
1 garlic clove, minced
½ cup olive oil
½ lemon, juiced
1 brioche bun
1 tablespoon butter, melted
4 ounces ground beef (80% lean, 20% fat)
2 slices sharp cheddar cheese
1 leaf butter lettuce
2 thin slices tomato
1. Preheat the oven to broil on high.
2. In a small pan, bring the apple cider vinegar, sugar, and salt to a boil over high heat, whisking until the sugar and salt have dissolved.
3. Remove the pan from the heat and add the sliced onion, making sure they’re fully submerged in vinegar. Set aside.
4. In a small bowl, add the egg yolk, mustard, and garlic and whisk to combine. Slowly add the oil in a steady stream, whisking to combine, ensuring that the aioli is emulsifying. Once the aioli is thick, add in the lemon juice and whisk until creamy. Add salt to taste, and set aside.
5. Brush the brioche bun with the butter and transfer to a baking sheet. Transfer the baking sheet to the oven, and broil for 1½ minutes, until the bun is golden brown.
6. Smear the top bun with aioli, and set aside.
7. Heat a cast iron pan over medium-high heat. Once the pan is hot, brush with oil.
8. Place the mound of ground beef on the pan, and immediately press flat with a spatula. 9. Season with salt and pepper, and cook for about 2½ minutes.
9. Flip the patty over and top with cheddar cheese and cook until the cheese is melted and the patty is browned, about 2 minutes.
10. Transfer the patty to the bun and top with pickled red onions, butter lettuce, and tomato slices.