Use a garlic mincer......
use a food processor
First you should chop up the onio into small peices. as small as possible. then you might want to squish them. i spoon can work. use something that you can smash them with. the onions wont turn out as slushy as it would when you blend it but it will turn out mushy. thats all i got
I think you either left off the gasket on your blender, or you put it on wrong. Take the blender apart and find the rubber ring at the bottom, which comes out. Wash out your blender and reassemble it, this time make sure you have the gasket facing the right way and in the proper position. Then, try again.
Anyhow, I am sure that chopping your onion up finely will work just as well. The ancient Koreans who invented kimchi certainly did not have blenders.