My kitten ate sauce with onions and has diarrhea. What should I do?

I suspect the sauce is causing the diarrhea, and not likely the onions. There likely wasn't enough onions to cause issues. I can give you a home remedy for Maru to see if it helps. Withhold food for 6 hours, then feed a bland diet of boiled white meat chicken (no bones) in small amounts several times a day for a couple of days. Then slowly add back in his normal diet. If the diarrhea continues on the bland diet, he starts vomiting food, stops drinking water, starts vomiting water, or becomes very lethargic, he will need to see a vet right away. Best of luck.