Preheat oven to 350 degrees F.
Combined softened butter with Italian seasoning and parsley; set aside half.
Very lightly toast all twelve slices of bread; set aside four.
Spread half of the seasoned butter onto one side each of eight slices of toast.
On a cutting board or clean work surface, turn four toasts butter-side-down, spread tops with 1 tsp. mayonnaise each, then layer two slices each of provolone, ham, and genoa salami, and one slice each of capicola and pepperoni.
Spread 1 tsp. mayo onto plain side of the other four toasts, and layer butter-side-up onto the sandwiches. Arrange sandwiches onto one half of a prepared baking sheet.
Butter the remaining four slices of toast on both sides with reserved seasoned butter; arrange on the other half of the prepared baking sheet.
Bake sandwiches and toasts in oven for 4-5 minutes; flip and bake 3-4 minutes more until cheese is melted, sandwiches are warmed through, and toast is golden-brown. Check occasionally and remove toasts sooner if getting too dark. Remove from oven and set aside.
Onto the four plain toasts, layer tomato, lettuce, onion, and olives, along with any additional antipasto ingredients and remaining mayo, if desired. Stack hot Italian sandwiches on top.
Divide club sandwiches into quarters, securing with each with toothpicks, and serve immediately.