In a large heavy-bottomed pot, melt butter over high heat. Once butter is melted and foamy, add green onion, garlic, and bay leaves; cook, stirring frequently, until green onions are softened, about 5 minutes.
Reduce heat to medium; add flour, and incorporate into mixture with a flat-bottomed spoon. Add crawfish and salt, black pepper, and cayenne pepper to taste, stirring well until combined.
Increase heat to high, and cook, stirring frequently, for 3 to 5 minutes. Mixture will become thick. Slowly add warm stock, and bring to simmer, stirring frequently. Taste for seasoning and adjust if necessary. Add a dash of hot sauce (if using). Serve over white rice, and garnish with green onion.
Recipe by Louisiana Cookin' at https://louisianacookin.com/st-mary-style-crawfish-etouffee/