Heat butter in a heavy-bottomed skillet, dutch oven, or cast-iron skillet over medium heat. Once the butter melts, add all-purpose flour, and mix with a whisk or wooden spatula until smooth.
Cook, stirring continuously until the roux reaches a peanut brown or caramel color. Switch off the heat, and continue whisking until it turns a shade darker. Set it aside.
Heat another saute pan over medium-high heat, add butter, once it melts, add diced onions, bell pepper, and celery along with minced garlic. Cook until the vegetables soften.
Add the prepared roux, mix well, and cook for another minute.
Add 1 cup of stock, and mix, and cook for 1-2 minutes, until the mixture thickens.
Then add the remaining 1 cup of stock and mix well. Add Creole/Cajun seasoning along with cayenne pepper, Kosher salt, 2 bay leaves (if using), and 1-(14.5) oz can of no-salt, diced tomatoes (only for a Creole version). Mix well, cover with a lid and simmer for 10-15 minutes, stirring occasionally in between.
If using regular Cajun or Creole seasoning, hold off the salt, since these already contain salt, taste and adjust in the end.
Add crawfish tail meat, mix well, cover, and simmer over low heat for about 5 minutes. Do not overcook the crawfish.
Check for seasonings, adjust with salt if required. If the etouffee is too thick, add the remaining 1/2 cup of stock, and heat it through. Sprinkle parsley and mix well.
Serve with steamed rice, sprinkle some more chopped parsley, and scallion greens. Add a few dashes of hot sauce. Enjoy!