Unlike its name might suggest, a cat sandwich is in fact entirely free from small feline creatures.
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.
Sam: Christ I sure am hungry. I could go for a fresh Cat Sandwich right now!
Rosie: That's sick! You eat poor little kitties!?
Sam: No, silly ho! It's a cheese-avocado-tomato sandwich.
Rosie: Eww, avocado...