In a large bowl, mix together the cheeses and the green onion. Stir in the mayonnaise and sour cream. In a separate small bowl blend the butter and garlic until smooth. Add the butter mixture to the cheese mixture.
Preheat broiler. Slice loaf of bread in half horizontally, lay crust side down. Spread cheese mixture over the bread. Place under the broiler until nicely browned, about 3 to 5 minutes.
Remove from broiler and let sit for 5 minutes until cool enough to handle.
Sprinkle freshly grated Parmesan cheese over loaves. Slice the bread with a bread knife. Serve with or without marinara sauce for dipping.