In a shallow Dutch oven, melt the butter over moderate heat. Whisk in the flour and stir constantly until the mixture turns a golden brown color.
Add the diced onions and stir until they caramelize. Then, add the celery, green bell pepper, red bell pepper, and half of the green onions. Cook for about 25 minutes, stirring occasionally. Add the minced garlic and cook for another 5 minutes.
Pour in the seafood stock and add your seasonings (garlic powder, salt, and pepper). Stir well, then add the crawfish. Cover and let it cook on low-medium heat for about 30 minutes, stirring occasionally to prevent sticking.
Add the remaining green onions about 10 minutes before serving. Taste and adjust seasonings as needed.
Spoon the crawfish étouffée over warm white rice and enjoy!