Heat the butter in a large skillet over medium-high heat. Add the flour and cook, stirring often, until the roux is pale brown, about 10 minutes. Add the celery, onions and bell pepper. Cook until the onions are soft and translucent, 3 to 5 minutes. Add the garlic and cook until fragrant, 45 to 60 seconds.
In small bowl, combine the stock and tomato paste. Add the stock mixture to the skillet and bring to a boil, stirring constantly. Reduce the heat to medium and simmer until slightly thickened and the flavors have married, about 10 minutes.
Add the crawfish, parsley and cayenne pepper. Cook until heated through, about 5 minutes. Taste and adjust for seasoning with salt and pepper. Spoon over hot rice. Garnish with scallions and serve immediately.