Slice French bread in half lengthwise and scoop out the inside of the loaf. Set aside.
In a large skillet, melt butter over medium-high heat. Sauté crawfish, onions, celery, bell peppers and garlic 15 minutes. Blend in dry mustard and mayonnaise. Add cheeses and blend until melted.
Spread crawfish mixture inside the bread then put halves back together. Butter the top of the loaf, wrap it in foil and bake on a barbecue pit or in a 350˚F oven for 20 to 30 minutes. Cut bread into slices and serve hot.