Melt the butter in a large Dutch oven set over medium heat. Add the flour and stir continuously to make a roux. Stir the roux over medium heat until the color of peanut butter, 5 to 7 minutes. Add the onions, bell peppers, celery, and garlic to the roux, and cook, stirring often, for 10 minutes.
Add the tomatoes to the pot and season with the bay leaves, salt, cayenne, and 1 tablespoon of the Essence. Cook the tomatoes for 2 to 3 minutes and then whisk in the shrimp stock.
Bring the mixture to a boil, and reduce to a simmer. Cook the etouffee, stirring occasionally, for 45 minutes. Season the shrimp with the remaining tablespoon of Essence and add them to the pot, stirring to evenly distribute. Cook the shrimp for 5 to 7 minutes, or until they are cooked through. Add the chopped parsley to the pot and stir to combine.
Serve immediately over steamed white rice and garnish with sliced green onion tops.
Place the shrimp shells and heads in a large colander and rinse under cold running water for several minutes.
Combine the shrimp shells and remaining ingredients in a heavy 6-quart stockpot, add 4 quarts water, and bring to a boil over high heat. Skim to remove any foam that rises to the surface. Reduce the heat to medium-low and simmer, uncovered, for 45 minutes, skimming occasionally.
Remove the stock from the heat and strain through a fine-mesh sieve into a clean container; let cool completely.
Refrigerate the stock for up to 3 days or freeze in airtight containers for up to 2 months.