In a bowl, combine mustard, ketchup, horseradish, lemon juice, paprika, white pepper, and cayenne pepper; mix well. Season with salt.
Slowly add oil while whisking vigorously. If needed, add more cayenne pepper and salt, adjusting to taste.
Add celery and scallion; mix well.
Cover and refrigerate until chilled, at least 30 minutes.
To prepare Fried Green Tomatoes:
Heat oil in a sauté pan over medium heat.
Whisk together egg and milk. Dip tomato slices in egg mixture, then coat with cornmeal.
Carefully place tomato slices in sauté pan and cook SLOWLY until golden brown on bottom, then turn and brown the other side. (Note: If tomatoes are cooked too quickly, the outside will brown before the inside is cooked.).
To serve, arrange greens on an individual serving plate. Place fried tomato slices next to each other on the bed of greens. Top each slice with three or four shrimp. Spoon remoulade sauce over shrimp.