Heat the butter over medium heat in a saucepan until it bubbles, then stir in the flour and make a light roux. Stir constantly until the texture changes, but don't allow the roux to brown.
Add the garlic and stir for about 30 seconds. Then lower the heat to the lowest setting and stir in the brandy. After about a minute, add the warmed half-and-half, and whisk until the sauce thickens. Add all the other ingredients except the crawfish and simmer, stirring once or twice, for about two minutes.
Fill ramekins or small au gratin dishes loosely with crawfish tails. Top with 3 Tbs. of the sauce. Place the dishes into a preheated 400-degree oven. When the sauce begins to bubble and brown at the edges, the dish is done. Serve immediately. Make sure French bread is on the table to help get the sauce.