Finely dice the onion, pepper and celery and finely dice the garlic. Finely slice the white and pale green part of the scallions and set aside for later.
Warm the butter in a pan over a medium heat and add all of the chopped vegetables (apart from scallions). Cook for a few minutes until they are all softening, add garlic, then continue to cook a couple minutes until the onions are translucent.
Stir through the paprika, thyme, oregano, pepper, cayenne, salt and tomato paste. Mix in and cook a minute then add the flour. Stir through and cook, stirring now and then, for a couple minutes. You should notice a slight nutty smell and it darkening slightly, but take care that the mixture doesn't burn. You may need to reduce the heat a little.
Add the stock, mix well and scrape away anything that is stuck to the bottom of the pan. Bring to a simmer. Reduce the heat and simmer for 10 minutes or so to thicken a little. Add the Worcestershire sauce and crawfish, cook for another 2-3 minutes to warm the crawfish through (assuming pre-cooked, as is more common - give a bit longer if they are raw to ensure cooked). Check and adjust seasoning, if needed, then serve over rice, garnished with scallions.