Courtesy of the Vargas family.
Sprinkle salt and sugar over onion slices and let stand for 10 minutes. Meanwhile, heat a cast iron skillet over medium high heat. Place onion slices in pan and cook until softened, about 10 minutes. Let cool.
In a large skillet, melt butter over medium heat. Cook green onions, cilantro, garlic, and cumin for about 3 minutes, or until softened and let cool.
In a large bowl, combine ground beef with onion mixture and season with salt and pepper. Chill 15 minutes minimum; shape into burger patties.
Grill burgers over medium high heat until no longer pink inside. Top each burger with a cheese slice; melt, and place cheeseburger on bun bottoms. Spread bun tops with grainy mustard, top burger with lettuce, tomato, charred onions and pickle before serving.