Boil shrimp with onion, garlic and 1/2 teaspoon cayenne. Peel and devein shrimp. Coarsely chop shrimp and set aside.
Combine onion, garlic, cayenne pepper, Neufchâtel cheese, sour cream, mustard, Tabasco sauce, minced garlic, white pepper, Worcestershire sauce, salt, cayenne pepper, and ketchup in a mixing bowl or food processor. Mix well; add shrimp and green onions. Allow to chill at least one hour before serving.