1. In a cast-iron skillet, melt butter. Whisk in flour and cook for 2 to 3 minutes, stirring constantly to form a blond roux.
2. Reduce heat and add onion, garlic, and cajun spice. Cook until onion is translucent, about 3 to 5 minutes. Add tomatoes and cook another 2 to 3 minutes. Whisk in seafood stock and bring mixture to a boil, then reduce heat to a lazy simmer. Cook for 30 to 40 minutes.
3. Add crawfish tails and cream and cook just until crawfish are heated through, 2 to 3 minutes. Taste and adjust seasoning with salt and pepper as needed. Serve with white rice and sliced green onions.