The "Cat" in a Cat Sandwich is actually just an acronym for the sandwich's filling; Cheese, Avocado and Tomato. (With pepper on top for extra deliciousness)
The fillings should all be sliced, with the cheese on the bottom, tomato in the middle and avocado on top.
Rosie: That's sick! You eat poor little kitties!?
Sam: No, silly ho! It's a cheese-avocado-tomato sandwich.
Rosie: Eww, avocado...