Cook the burgers. Heat a swirl of oil in a cast iron skillet over medium heat. Add the patties and cook until the bottom is browned and juices start to pool at the top of the burger.
Make the aioli. Meanwhile, whisk together the mayonnaise and fresh herbs. Add a squeeze or two of citrus juice to taste, then season with salt and pepper.
Assemble the burgers. To make the burgers, smear both sides of the buns with herbed mayo. Top with onions and tomato slices, the burger with cheese, avocado slices, and sprouts.