Preheat your oven to 350°F (175°C).
In a large skillet, melt the butter over medium heat. Add the diced onion, diced celery, diced red bell pepper, and minced garlic. Sauté for about 5-7 minutes, or until softened and fragrant.
Stir in the crawfish tails and cook for an additional 3-4 minutes until heated through.
In a mixing bowl, combine the sautéed mixture with dry mustard, mayonnaise, mozzarella cheese, and cheddar cheese. Mix well.
Slice the French bread in half lengthwise and hollow out the center of each half.
Spread the crawfish filling evenly between the two halves of the French bread.
Place the filled bread halves on a baking sheet and bake for about 15-20 minutes, or until golden.
Remove from the oven, let cool slightly, slice into portions, and serve warm.