I have found that the best food for fattening up a chuck is Mazuri Tortoise chow. My Angel Island chucks go crazy for the stuff. I soak for about a minute and then give it to them. I know that a number of Uromastyx breeders also use it as a staple food for their lizards. You generally cannot find it in stores, but some folks over in the Kingsnake classifieds sell it.

Good luck!

Bart