I think the biggest concern with the fat would be diarrhea.
Regarding the onion powder. watch for lethargy, ataxia (loss of balance), anemia (pale gums).....I think there may be other symptoms but these would be the most obvious.
See if he wants lunch, then if he does, maybe just some plain lean meat if you have it.
Actually...I've just been doing a little reading.
If this were me, I would call the poison control center for advice.
The number is
1-888-426-4435. There is a fee, and you will be able to speak to a vet and be given a case number.