Preheat the oven’s broiler.

Mix together mayonnaise, Parmesan cheese, and garlic powder in a medium bowl until thick and well combined.

Slice loaf in half lengthwise and spread the cut-side of each piece with mayonnaise mixture. Place halves on a medium baking sheet.

Cook under the preheated broiler until mayonnaise mixture is bubbling and golden brown, 2 to 5 minutes; check frequently to avoid burning.