Best Storage:
Cool well ventilated place - eg. Cupboard
Store away from potatoes (if they are side by side they will cause eachother to spout). If they do sprout, you can cut this section off and pop it in your garden to grow! The rest of the onion is fine to eat. Place cut onion in an airtight container in the fridge or use a silicone food saver over the cut end.
Freezer?
Yes
Simply chop and divide into portion sizes for later use. Freeze in a freezer safe container that will not leak odour to other foods. Use within 3-6 months for maximum flavour.
Use it up!
Add frozen onion directly to your cooking. Once frozen it’s not great for addition to salads as it does loose its crisp texture.