Preheat oven to 350°F.
Line a cookie sheet with foil.
In a small bowl stir together mayo, mustard and horseradish sauce, set aside.
Cut loaf of bread in half horizontally and place cut side up, on a foil lined cookie sheet.
Spread mayo mixture onto both halves of bread.
Divide pepperoncini pepper slices onto both halves of bread.
Top each half with ham, turkey, salami and pepperoni slices.
Top each half with cheese slices.
Sprinkle with red pepper flakes if desired.
Bake in the oven for 10-15 minutes until heated through and cheese is hot and melted.
Turn oven temperture to broil and toast sandwiches until cheese starts to bubble and turns golden brown along the edges.
Remove from the oven and drizzle with olive oil and balsamic vinegar if desired.
Top one half with sliced tomatoes and lettuce if desired.
Place the two halves together and cut sandwiches in half to serve.