Warm a heavy-bottomed 6-quart (6 L) Dutch oven over medium heat for 2 minutes, then add the oil and heat for 30 seconds. Add the flour and cook, stirring continuously with a wooden spoon, for 15 to 20 minutes, until the roux is a little darker than cafe au lait, closer to the color of peanut butter. Don't walk away-focus on stirring your roux.

Add the onions to the roux and stir to combine. (Be careful to avoid splattering the roux when adding the onions-this is when folks sometimes burn themselves.) Cook, stirring occasionally, until the onions are soft and translucent, about 20 minutes.

Add the bell pepper, celery, and bay leaf to the onions and stir to combine. Reduce the heat to low, cover, and let the vegetables smother together until very soft, with no bite remaining, about 20 minutes, stirring halfway through to make sure nothing is burning and to ensure even cooking. Taste a piece of bell pepper and a piece of celery-if they're ready, there should be no more crunch to them. If they still have a lot of bite, cook the vegetables for another 5 minutes, then test again.

Meanwhile, put the crawfish in a large bowl and season with the black pepper, cayenne, hot sauce, and salt. Set aside at room temperature to marinate while the vegetables cook.

Add the stock to the vegetables and stir. Bring the liquid to a boil over medium-high heat, then reduce the heat to maintain a simmer and cook for 25 minutes to let the flavors marry. You want to reduce the liquid by a quarter or half, depending on how thick you want your stew. Add the seasoned crawfish and simmer for 5 minutes longer.

Taste the stew. Does it need more salt or pepper? Add some. Does it need more heat? Add cayenne. Does it need acid? If so, add hot sauce. Adjust the seasoning to your liking, then stir in half of the parsley and green onions and save the rest for garnish.

Serve the stew garnished with the remaining parsley and green onions, with biscuits alongside.