Add lime juice, sea salt, minced red onion, and minced jalapeño (if using). Using a fork, roughly mash to a slightly chunky texture — leave some small pieces for texture. Taste and adjust salt or lime as needed. Cover and set aside at room temperature.

Herb Aioli

In a small bowl combine mayonnaise, lemon juice, grated garlic, chopped parsley, chives, dill (if using), salt, and pepper. Whisk until smooth and well combined. Refrigerate until ready to use so flavors marry.

Burger & Assembly

Divide the ground beef into four equal portions (about 4 ounces each). Gently form each portion into a loose ball, but do not overwork the meat — you want it to smash easily.

Heat a large cast-iron skillet or griddle over medium-high heat until very hot. Add the vegetable oil and spread to coat.

Place one beef ball on the hot surface. Using a stiff spatula or a second flat spatula (and a piece of parchment or wax paper if desired), press the ball down firmly to form a thin patty about 1/4 inch thick. Repeat with remaining balls, leaving space between patties. Work quickly so the pan remains hot.

Cook the patties without moving for 2 to 3 minutes, until deeply browned at the edges and juices start to bubble on the surface. Sprinkle each patty with a pinch of kosher salt and a grind of black pepper.

Flip each patty and immediately place a slice of Swiss cheese on top of each. Cook for another 1 to 2 minutes until cheese is melted and the patty is cooked through but still juicy (internal temp ~ 155°F/68°C for medium).

While patties finish, butter the cut sides of the buns and toast them face-down in a separate skillet or under a broiler until golden, about 1 to 2 minutes.

Assemble burgers: spread about 1 tablespoon of herb aioli on the bottom bun, add a handful (about 1/4 cup) of shredded lettuce, set the hot Swiss-topped patty on the lettuce, spoon 2 to 3 tablespoons of smashed avocado on top of the cheese, then finish with the top bun smeared with a little more aioli if desired.

Serve immediately while hot. Provide extra lime wedges and additional smashed avocado or aioli on the side if desired.