In a bowl, combine the chopped shrimp and shrimp paste. Add the egg, breadcrumbs, minced garlic, chopped parsley, lemon juice, Old Bay seasoning, salt, and black pepper.
Spread the lemon mayonnaise on the buns. Add lettuce leaves, a shrimp patty, tomato slices, and red onion slices. Top with the other half of the bun and serve.