Cut the focaccia into quarters and reserve three-fourths for more sandwiches later. Cut your piece of focaccia down the middle on the bias, split it open, and drizzle olive oil all over the inside of both sides of bread.
Heat a large skillet and add olive oil to the pan. Once the oil is hot, add one-half of the focaccia and toast until golden and nice and crunchy. Just toast the one side, then remove to a plate, crispy side facing up. Repeat with the other half.
Take one slice of mortadella, then add one slice of cheese. Then add another mortadella slice and a slice of cheese on top of that. Repeat until you have a stack of four slices of mortadella and three slices of cheese in between. Heat the meat pancake over medium heat until the cheese starts to melt. Leave warm and melty in the pan.
Spread mayo over both sides of the toasted focaccia, followed by the mustard. Place the lettuce on the bun bottom and top that with the onion. Place the cheesy mortadella on top and finish with the bun top, then slice down the middle, exposing the layered cheesy goodness. Spear a dill pickle with a toothpick, then spear the top of a sandwich half. Repeat with the other sandwich.