In a small bowl whisk together the mayo, mustard, ketchup, and relish. Refrigerate until ready to use.
3 tablespoons mayo, 1 teaspoon mustard, 1 teaspoon ketchup, 2 tablespoons relish
Add the ground beef to a medium bowl, then add salt, pepper, onion powder, garlic powder, and Worcestershire sauce. Use your hands to gently incorporate everything into the ground beef. Divide the meat in half and form each into a rectangular shaped thin patty about the size of the pieces of bread you are using.
¾ pound ground beef, kosher salt, black pepper, ¼ teaspoon onion powder, ¼ teaspoon garlic powder, 1 teaspoon Worcestershire sauce
Heat a cast iron skillet over medium high heat and cook each patty for a few minutes on each side until browned and cooked through to your desired liking (see note). Remove from heat.
To layer each grilled cheese add 2 slices of American cheese to one slice of bread, top with one of the patties, then spread half of the sauce on to the patty. Top the sauce with 1 tablespoon each of chopped onion and pickles, then top with 2 additional slices of American cheese. Top with another piece of bread. Repeat to make the other sandwich.
4 slices white bread, 8 slices American cheese, ⅛ cup onion, ⅛ cup pickles
In a large skillet, melt butter over medium heat. Add grilled cheeses, pressing down lightly, and cook for 3-5 minutes until golden. Flip and continue cooking until cheese is melted and bottom piece is golden. Sprinkle with sesame seeds and serve with ketchup.
2 tablespoons butter, sesame seeds, 1 teaspoon ketchup