Preheat an oven to 400 degrees F (200 degrees C).
Slice bread in half horizontally. Using your fingers pull out most of the soft bread to form a hollow shell. Save pulled-out bread for another use.
Mix together butter, mushrooms, cheese, green onions, and garlic. Spread the mixture on both cut sides of bread. Place the bread, cut sides up, on baking sheet.
Bake in preheated oven until the cheese has melted, about 10 to 15 minutes. Cut in wedges to serve.