Combine the lemon juice, sweet onions, green onions, celery, horseradish, mustards, and ketchup in the bowl of a food processor. Process on high for 30 seconds. With the machine running slowly drizzle 3/4 cup of the vegetable oil into the food processor until well incorporated and slightly thickened. Add the parsley, 1/2 teaspoon of the salt, and the cayenne pepper. Pulse to combine. Use immediately or store in the refrigerator in an airtight container. (The remoulade will keep for several days in the refrigerator.)
Place the shrimp in a medium bowl and season with 2 teaspoons of the Essence and the remaining 1/2 teaspoon of salt. Melt the butter in a large saute pan over medium-high heat. Add the onions and peppers. Saute for 1 minute. Add the shrimp and continue to saute for 2 to 3 minutes. Add the remoulade sauce, bring the mixture to a simmer, and cook for 1 minute. Cover and keep warm over low heat.
Season the tomatoes with salt and freshly ground black pepper, to taste. Place the 1 cup of the flour in a small bowl with the cornstarch and mix well. In a separate bowl, whisk together the eggs and milk. In a third bowl, combine the remaining 2/3 cup flour, the cornmeal, and the remaining 2 tablespoons of Essence. Dredge each slice of tomato in the flour mixture. Dip each slice in the egg wash, letting the excess drip off. Finally, dredge each slice in the cornmeal mixture, coating completely.
Heat 1/2 cup of oil in a large saute pan over medium-low heat. When the oil is hot, pan-fry the tomatoes, in batches, until golden and crispy on both sides, about 1 1/2 to 2 minutes per side. Add the remaining 1/2 cup of oil to the saute pan as needed, preheating the oil before frying the tomatoes. Remove the browned tomatoes and drain on paper towels. Season lightly with salt, to taste.
To serve, lay 3 tomatoes in the center of each plate. Spoon 1/4 of the warm sauce over each plate of tomatoes. Garnish with green onions and serve immediately.