In a heavy bottom pan melt butter and add flour to make a roux. Stir constantly over a low heat until it takes on a beige color and loses the floury taste; about 30 minutes. When the roux is done, stir in vegetables, bacon and parsley. Add in beer, clam juice, bay leaf, seasonings and half-and-half. Stir, and cover pot. Let simmer for 30 to 45 minutes. Nearing the end of cooking time, add in precooked crawfish and serve over rice.