>>> is there anything that you should NEVER feed a cat?
Onions can kill cats but they'd need a lot of them (either all in one go or spread out over a period of time) before any real damage could occur. (There'd be no need to worry just because a cat licked up a bit of onion gravy).
See here for other things which cats shouldn't have:
http://www.petmd.com/cat/emergency/poisoning-toxicity/e_ct_human_food_poisoning
Every plate, saucepan and frying pan in my house is 'pre-treated' (by four cat tongues) before it goes into the washing up bowl.
If I'm preparing a meal with cheese I automatically cut off lots of small pieces, and throw them onto the floor, so that my two cheese-loving cats can join me.
If I'm eating, say, sausages, I'll keep throwing small pieces on to the floor for the cats.
If I buy a chicken and stuffing sandwich from the supermarket for my tea, I often end up eating a stuffing sandwich, while the cats enjoy the chicken.
My own diet is often determined by what the cats can have. (I'll reject a nice-looking ready meal in a supermarket, for example, if I see that it's got lots of onions in, as the cats won't be able to lick out the container and clear my plate).
I'm always on the lookout for food which can feed both me and the cats. For example, a pack of four chicken pieces from the rotisserie counter in Asda will feed my four cats (and probably the two from over the road as well), while still leaving me enough to make a chicken curry for myself.
Oh, yes, they do get cat food as well. They've always got two types of 'pouched' food (one with gravy and one with jelly) available to them, along with with premium cat biscuits (plus, of course, lots of clean water).
. . . and even with all that lot, they still look forward to their daily treat of Dreamies ;-)