You can start thinning your carrots out by pulling up alternate ones and using either in salads or lightly cooked. This will give the remaining roots more space to grow in. The rest of your carrots can remain in the ground until just before it is likely to get frosty when you should life and store in a cool dry area. We never have enough to store though. As for the onions you can pull the odd one as needed, once the neck of the stem folds over and the leaves start to yellow lift them all and lay on the soil to dry out for a few days. Store somewhere cool and dry and use the onions where the neck did not close properly first as these will not keep well.