In a large Dutch oven or heavy pot, heat the oil over medium heat until it ripples, then sprinkle in the flour. (Test the oil by dropping a pinch of flour into the pot. If it sizzles, it is hot enough). Stirring constantly, cook flour and oil together until the mixture toasts and turns golden brown, 2 to 3 minutes.
Step 2
Add onion, garlic, bell pepper, celery and salt and stir. Cook, stirring, over medium heat until the onion is translucent, about 3 minutes. Add tomatoes, stock, bay leaf, dulse, Creole seasoning, and Old Bay. Stir well and bring the pot to a simmer. Add mushrooms and simmer, stirring occasionally, until tender, about 20 minutes. Taste and adjust the seasonings with salt and Old Bay.
Step 3
Serve étouffée spooned over rice, garnished with lemon and parsley.