Heat a large wok on high until near smoking, then coat with oil. Add bell pepper, onion, and celery, and sauté until translucent. Add garlic and sauté until fragrant. Add crawfish and Cajun seasoning and toss until homogenous. Add chicken stock (or beer) and cover. Steam about 5 minutes and uncover. Add seasoning, hot sauce, and cornstarch slurry. Sauté until sauce thickens and crawfish look glossy. Transfer to plate, and garnish with scallions and fried garlic (found at any Asian grocery store).