With Boo it was trial and error. It was pretty easy because I switched to lamb and rice and he stopped throwing up all the time.

With Jemma, it's a bit harder. She's allergic to grains more, and I'm not sure I've figured them all out yet. For sure she's allergic to wheat and corn because one wheat/corn cookie and her ears flare up, and ragweed makes her feet and anal glands itchy, but there could be other things too...
Oh, but this food would be a good place to start since the grains (or grain-replacements

) are very unusual.