Cat Health Questions? Ask a Cat Vet for Answers ASAP.
Yes onions, cooked or uncooked, are toxic to cats. Amounts as little as 5 grams (about a teaspoon) of onions per kilogram (2.2 pounds) of body weight can be toxic. Cats do not have an enzyme that is needed to properly digest them and as a result the toxins build up causing the red blood cells to lyse (break apart) and this leads to red colored urine and possibly kidney failure, low oxygen in the tissues causing the heart to race, increased respiratory rates, depression and possibly death if treatment isn't started. The effect can be accumulative too meaning that if a small amount of onions are fed frequently they can still be as toxic as a large amount ingested at once.
Fennel is listed as toxic by the ASPCA. Symptoms of toxicity include vomiting, diarrhea and allergic type reactions such as hives or facial swelling and difficulty breathing. I wasn't able to find a toxic amount.
I wouldn't chance feeding either plant to my pets, even in small amounts.
Please let me know if you have further questions.