Yes onions can make you fart if you eat enough of them whether raw or cooked.
You can eat onions either raw or cooked and they will still sometimes cause some people to fart because the onions cause gas buildup in the stomach and intestines.
Onions also contain fructose which is sugar that gets broken down in the intestine and when the fructose sugar gets broken down in the intestines it can produce gas in your stomach and intestines which can lead to farting.
Onions can also make you burp as well and cause stomach pain if eaten too much because the gas they cause in your stomach.
Eating onions raw or cooked is healthy though but you get less gas buildup from the onions if the onions are cooked.