Divide meat into 2 even portions, about 4 ounces (115g) each. Form into two 4-inch patties. Season patties liberally with kosher salt and pepper, transfer to a plate, and set aside.

In a 10- or 12-inch cast iron skillet, heat butter over medium heat until melted. Place buns cut side down and toast, one at a time, until lightly golden, 2 minutes. Set aside.

In the now-empty skillet, heat oil over medium heat until shimmering. Add onions, season with 1 ½ teaspoons salt, and cook, stirring occasionally, until softened and translucent, about 5 minutes.

Using a heat-proof spatula or wooden spoon, push onions to one side of the skillet and add the patties to the empty side of the skillet. Cook without moving, until a dark crust develops, about 3 minutes. Using a spatula, carefully flip patties and cook until dark brown crust develops on second side, about 1 minute. Using the spatula, stir in the onions and sazón while breaking up patties until meat is chopped up into bite-sized pieces no larger than ½ inch. Cook until meat is no longer pink, 2 minutes. Season with salt and pepper to taste.

In the pan, divide ground beef and onion mixture into 2 individual portions the length of the rolls. Shingle 2 slices of American cheese over each portion; allow cheese to melt, 1 to 2 minutes.

To Assemble: On a clean work surface, lay out a 16-inch wide piece of aluminum foil or parchment paper. Place toasted roll, cut side up, onto foil at a bias. Dress bread with desired condiments, fill with meat and onion mixture, and top with tomato slices and lettuce. Close sandwich and wrap tightly with foil. Using a serrated knife, halve the sandwich crosswise, and serve immediately.