This classic patty melt has white Cheddar cheese, caramelized onions and a thin all-beef patty sandwiched together on toasted marbled rye. Serve with mustard, pickle chips and any of your family's favorite condiments for an easy meal.
For the caramelized onions: Melt the butter in a medium skillet over medium-low heat. Add the onions and garlic. Cook, stirring occasionally, until the onions are very tender and caramelized, 15 to 18 minutes. Add a drizzle of vegetable oil if the pan is getting too dry. If the bottom of the pan is getting too dark before the onions are cooked through, deglaze with 1 tablespoon of water and scrape any brown bits from the bottom of the pan. Once caramelized, add the vinegar and cook until it has fully absorbed, 1 to 2 minutes. Set aside to cool slightly.
For the patty melts: Meanwhile, preheat a large cast-iron griddle over medium-high heat. Mix the ground beef, seasoned salt and a few good grinds of pepper in a large bowl until combined. Use your hands to shape the meat mixture into 4 even balls, shaping them to match the shape of your bread, then sprinkle with a pinch of kosher salt and pepper. Use the palm of your hand to smash the balls into a thin and slightly oval patty (to fit the size of the rye bread). Place the patties on the hot griddle and cook undisturbed until the first side is well browned, 3 to 3 1/2 minutes. Flip the burgers and cook until the other side is well-browned and they are medium doneness, about 2 minutes more. Transfer to a plate. Reduce the temperature to medium-low and wipe the griddle clean with a few paper towels.
Spread 1 tablespoon of the mayonnaise on all the slices of bread, then place mayo-side down on the cutting board. Top the naked side of the bread with a dollop of Dijon if using. Top 4 of the bread slices with 2 slices of cheese, a patty, some caramelized onions and 2 more slices of cheese. Top with remaining bread slices.
Place all 4 sandwiches on the griddle mayo-side down and cook until the bread is toasted and golden brown and the cheese is completely melted, about 2 minutes per side. Cut each sandwich in half crosswise and serve with mustard and dill pickle chips.