Place the crawfish tails in a bowl and sprinkle with some creole seasoning and set aside. Chop the yellow onions, peppers, celery and set aside in a bowl for later. Chop the green onions and set aside. Chop the garlic and set aside. Mix the Herb and Spice Blend in a small bowl. Set the butter aside to soften and the flour for the roux.
Place about 3 cups of the seafood stock in a stock pot to warm (reserve the remainder if needed to thin the etouffee at the end).
Place a cast iron pan over medium-high heat. Add 8 tablespoons of butter until bubbly, then add the flour and whisk continuously to make a medium roux, the color of light chocolate milk. (See Recipe Notes)
Add the onions, celery and bell peppers (the Trinity) to the roux, and continue to stir still over medium-high heat until smooth and moist, about 15 minutes.
Add about half the green onions and all the garlic; continue to sauté until aromatic-about 1 to 2 minutes; add about ½ of the Herb and Spice mix and both bay leaves and mix well to combine; continue to sauté about 5 more minutes.
While the veggie/roux mixture is finishing up, turn the heat to high under the stock pot and bring the stock to a rolling boil.
Bring the stock to a full boil in the stock pot. When the veggie/roux mixture is ready, mix it into the boiling stock with a slotted spoon, one spoonful at a time, stirring until the whole spoonful is fully dissolved. Continue this until all the veggie/roux mixture is incorporated into the stock, making sure to return the stock to a full rolling boil in between spoons. Set the cast iron pot aside, do not clean it.
Allow the gravy to remain at a high boil for about 5 minutes, then reduce the heat, cover the stock pot and simmer for 10 to 15 minutes, stirring occasionally.
Turn the heat to medium high under the cast iron Dutch oven used for the roux and add 2 tablespoons of butter. As the butter melts scrape up any fond that's stuck to the bottom of the pan.
When the butter is foaming, add about ½ to ⅔ of the reserved green onions (saving some to sprinkle on top of the etouffee when serving) and sauté a minute or two, then add the crawfish tails and about half of the remaining Herb and Spice Blend. Mix well until all the crawfish are coated with butter.
Sauté about 2 to 3 minutes stirring constantly; you’ll see some liquid developing from the butter and rendering the crawfish. (See Recipe Notes).
Ladle the gravy mixture into the crawfish tails and mix well. The liquid from sautéing the tails in the butter will thin out the gravy.
Mix well, taste for spice and add more if needed. When this comes to a heavy simmer, lower the heat to a slight simmer for about 10 minutes, covered, while the flavors all come together. If you think it is getting too thick, place the cover on the pan while it simmers; if it's still too thick, add some reserved stock to thin it out.
Add the remaining 2 tablespoons of butter and shake the pan back and forth, don’t stir, until the butter is melted into the etouffee-then give it one final gentle stir.
Remove the bay leaves and serve over white rice.