Combine butter, oil, and flour in a large cast-iron skillet over medium heat.

Use a cooking utensil such as a wooden (or silicone) spoon or spatula to stir the mixture until it forms a roux and the desired color is achieved. Golden to peanut butter color is most preferred.

Add onion, green and red bell peppers, and celery to the skillet and cook, stirring frequently until softened, about 10 minutes. The roux will appear to have been absorbed by the vegetables (see images for references).

Next, stir in garlic, thyme, and bay leaf; cook for 2 minutes more.

Add fire-roasted diced tomatoes (including liquid from the can) and Worcestershire sauce, and continue cooking for 5 minutes. 

Add seafood stock and stir to combine.

Stir in cleaned crawfish tail meat and simmer for 5 to 7 minutes, or until the crawfish is heated through.

Adjust for flavor and consistency if desired.