Directions 7 steps | 60 Minutes
Make the pickled red onions. Cut the onion into thin slices. In a small pot over medium heat, add the vinegar, water, sugar, salt and peppercorns. Whisk until the sugar dissolves and bring to a boil.
Turn off the heat and add the onions and garlic to the pot. Press down with a wooden spoon to submerge all the vegetables in the liquid.
Place a sheet of paper towel directly over the vegetables and lightly press down to saturate it in the vinegar mixture. This will keep the vegetables submerged. Allow to cool in the pot for about 30-45 minutes before transferring the onions to a jar and storing it in the refrigerator.
Spread the butter on the outer slices of bread and toast in a large nonstick skillet over medium heat. Set aside.
Slice four generous 1-inch thick cuts of meatloaf. If the meatloaf is cold, heat the meatloaf on both sides in the same nonstick skillet over medium-low heat. While the meatloaf is heating, prepare the lettuce by slicing, washing and drying the lettuce leaves. (See notes.)
Top the meatloaf slices with cheese and melt. (See notes.)
Assemble your sandwich. On a piece of toasted bread, slather on mayonnaise. Lightly season with salt and pepper. Add the meatloaf and top with crisp lettuce leaves. Top the lettuce with pickled red onions. Add ketchup and/or mustard, if desired. Enjoy!