Heat flour in oven on baking sheet at 350º for 30 minutes, stirring every 10 minutes. It should be light brown. This can be done days in advance.
Combine all spices and separate into two small bowls. Set out all remaining ingredients so they are ready to add. Rinse crawfish tails well and pat dry.
Melt 1/2 cup of butter in a cast iron skillet, slowly add flour and 1/2 of the spice mixture and cook until dark brown, add tomato paste. Add warm stock slowly, stirring constantly until it has thickened.
Melt remaining 1/4 cup butter in a separate skillet and saute vegetables, 1/2 at a time. Add the remaining spices to the second half of the vegetables. Add all of the sautéed vegetables and crawfish tails to the finished roux and cook for 1 minute. Garnish with green onion , parsley and lemon juice.
Serve right away over rice. You can make the roux in advance and then warm it while you are sautéing the vegetables.