Step 1Pickle the onion: In a small nonreactive bowl, combine the onion, vinegar, sugar, salt and several grinds of pepper. Set aside for 20 minutes. Drain, squeezing excess liquid from the onions and place the onions in a large bowl. Reserve the pickling liquid.
Step 2To the bowl of onions, add the radicchio, celery, tomatoes, pepperoncini peppers, olives, basil and oregano. Sprinkle over 3 tablespoons of the sherry vinegar pickling liquid, along with 2 tablespoons oil. Gently massage the mixture together and taste, adjusting the flavorings and seasonings as desired. Cover and refrigerate for 20 minutes before using to give the flavors time to marry. This makes about 1 quart slaw, which will keep, covered and refrigerated, for up to 3 days.
Step 1Spread 1 side of each of the slices of bread with mayonnaise. Top each slice with half of the cheese. On top of one slice, place the warm meatloaf slice, then the slaw. Top with the inverted second slice of bread to form a sandwich, pressing gently so everything stays together.
Step 2Heat a griddle over medium-high heat until hot. Meanwhile, spread the outside of one side of the sandwich with mayonnaise.
Step 3Invert the sandwich onto the hot griddle, and spread the second (top) side with mayonnaise. Grill the sandwich until the bread is crisp and golden-brown, and the cheese is starting to melt. Flip the sandwich and repeat with the second side. Remove from heat.
Step 4Slice the sandwich and serve immediately.