Allow butter and cream cheese to soften before starting. Mix mayo, butter and cream cheese. Do not use a blender, as the mixture will get soupy. Add lemon juice, garlic, worcestershire sauce, pepper, etc. to taste.
Mix in crab (or whatever) and onion. Of the choices, I feel crab is the best. Chill at least two hours, cover with cocktail sauce. This is an important part of the recipe.
I recommend Wakefield crab meat, Shrimp or Shrimp & Crab. I usually use Crosse & Blackwell's cocktail sauce, but feel free to use any kind that you trust. You can make your own simple cocktail sauce by mixing 3 parts catsup to 1 part horseradish and adding tabasco, garlic, worcestershire sauce to taste.
Serve over Triscuits (they're nice and strong, to survive heavy scooping) or veggies. Irish is the friend from whom I got this recipe.
NOTES: Incredible crab or shrimp dip -- My first encounter with this dip came at a New Year's party a few years ago. I was standing by the munchies table trying not to embarrass myself too much, when a woman came up to me and pointed out this dip. She explained to me that at a previous party she had lost control eating it and got a spoon!
It's incredibly rich, and requires a lot of will power not to eat too much. Yield: Serves 6 gluttons.