Make the Garlic Aioli: In a small bowl, whisk together mayonnaise, garlic, lemon juice, truffle oil, and a pinch of salt and pepper. Cover and refrigerate until ready to use.
Sauté the Mushrooms (Optional): Heat a little oil in a pan over medium heat. Add sliced mushrooms and sauté for 5–7 minutes, until golden brown and tender. Season with salt and pepper, then set aside.
Prepare the Burger Patties: Divide the ground beef into 4 equal portions and gently roll them into balls. Avoid overworking the meat to keep the burgers juicy. Season with salt and pepper.
Cook the Smash Burgers: Heat a large cast-iron skillet or griddle over medium-high heat. Add the meatballs to the skillet and immediately top with a handful of thinly sliced onions. Smash the meatballs flat with a heavy spatula or burger press until thin, pressing the onions into the meat. Cook for 1–2 minutes until the edges are crispy and browned.
Add Cheese: Flip the patty and sprinkle a generous amount of Parmesan on top. After about 1 minute, add a slice of Havarti and cook for an additional minute, until melted and cooked through.
Assemble the Burgers: Spread garlic aioli on the bottom buns. Place patties on top, add mushrooms if using, and close with the top bun.
Serve: Serve hot with fries or a side salad. For extra truffle flavor, drizzle a little more truffle oil on top.
