Melt in a large skillet or Dutch oven over medium heat:
Gradually whisk in:
Cook, stirring constantly, until the roux is almost as dark as milk chocolate, about 20 minutes. Stir in:
Cook, stirring, until the vegetables are softened, 5 to 6 minutes. The roux will continue to darken to a deep mahogany color. Add:
Stir well and cook for 1 minute more. Stir in:
Stirring constantly, bring the sauce to a simmer. Add:
Bring the liquid back to a simmer. Reduce the heat so the sauce bubbles gently, cover, and cook until the shrimp or crayfish are firm and pink, about 10 minutes. Add:
Season to taste with:
Serve with: