1
I need to buy an onion.
To sign this, point to yourself for 'I', sign 'need' by bringing a hooked 'X' hand downward, sign 'buy' by handing over imaginary money, and finally sign 'onion' near your eye.
2
Do you like onions?
Point to the person for 'you', sign 'like' by pulling your thumb and middle finger away from your chest, and sign 'onion'. Remember to raise your eyebrows and lean forward slightly to indicate a yes/no question.
3
This soup has a lot of onions.
Sign 'soup' by mimicking eating from a bowl, sign 'have', use the sign for 'many' or 'a lot', and finish with the 'onion' sign. You can use facial expressions to emphasize the large amount.