Make the garlic bread waffles: Heat a waffle iron to medium-high.
Place 4 pieces of sourdough bread in the waffle iron, brush with butter and sprinkle with garlic. Close and cook for 2 to 3 minutes until golden. Repeat with remaining slices of bread to make 2 large waffles total.
Assemble the sandwiches: Return 1 waffle to the iron. Top with chicken tenders, marinara sauce, oregano, pepper flakes, mozzarella and remaining waffle. Close and cook until cheese melts, about 3 to 5 minutes. Remove and slice into 4 sandwiches. Serve immediately.