For the remoulade: Whisk together the mustard, horseradish, paprika, yolk, lemon juice and zest and some salt and black pepper in a medium bowl. Gradually add the oil in a slow, steady stream while whisking constantly, until all the oil is incorporated and emulsified. Stir in the celery, onions, ketchup, vinegar, parsley, Worcestershire, garlic and hot sauce to taste. Cover with plastic wrap, and refrigerate for at least 1 hour and up to 24 hours to allow the flavors to meld.
For the fried green tomatoes: Put the cornmeal in a large shallow bowl and sprinkle with salt and pepper. Season the tomatoes with salt and pepper on both sides. Dredge the tomatoes in the cornmeal on both sides and tap off excess.
Heat 1/2 cup of the oil in a large saute pan over medium heat until it begins to shimmer. Fry half of the tomatoes until golden brown on both sides and just cooked through, about 1 minute 30 seconds per side. Transfer with a slotted spatula to a baking sheet lined with paper towels. Season with salt and repeat with the remaining tomatoes and oil.
For the shrimp: Combine the celery seeds, garlic powder, onion powder, paprika, salt and pepper in a mortar and pestle and crush into a somewhat fine powder. Toss the shrimp with the spice mixture.
Heat 2 tablespoons of the oil in a large saute pan over high heat until it begins to shimmer. Add the shrimp and cook until crusty and just cooked through, about 3 minutes (do this in batches if necessary with more oil; the shrimp should be in a single layer).
For assembly: Toss lettuce with a few tablespoons of the remoulade and season with salt and pepper. Divide the lettuce among the tomatoes and top each tomato with 3 shrimp. Drizzle with more of the remoulade. Garnish with parsley leaves.