Prepare the butter mixture by combining softened butter, grated parmesan, and garlic powder in a bowl. Spread this mixture on one side of each sourdough slice.
Assemble the sandwich by layering cheese on the unbuttered side of one slice of bread, followed by turkey cold cuts and another layer of cheese. Top with another slice of bread (buttered side up).
Heat a nonstick skillet over low heat and toast the sandwich for about 5-6 minutes per side until golden brown and the cheese melts.
In a separate bowl, mix mayonnaise with red grape juice vinegar, remaining parmesan cheese, garlic powder, Italian seasoning, and cracked pepper to create the creamy veggie dressing.
Toss shredded lettuce, sliced onions, and banana peppers into the dressing until well-coated.
Once toasted, fill the sandwich with the veggie mixture before serving.