Two Snakes said:

What do you need to do to onion plants while they are maturing in
order for them to grow into large slicing onions?

You need to choose the right varieties for your location, long day/spring
planted in the north, short day/fall planted in the south. ZThere are some
'day neutral' onions that can be grown north and south ('Super Star' is an
All American Winner in this class). Up here in Michigan you need to get your
onions in before May 10 if you want to ensure having good sized, well-cured
onions for storage.

Keep them weed free, space them far enough apart, water plenty and fertilize them
lightly while they are small. Cut back the water when the bulbs size up and
stop growing.

You can harvest green onions and green bulbing onions for fresh use through the
growing season. When the tops fall over and dry up, you can harvest for storage.

Medium sized onions need at least 4" spacing all around, large onions 6" and
the 'exhibition' type giant onions would like even more space.

--
Pat in Plymouth MI

Any technology distinguishable from magic is insufficiently advanced.
(attributed to Don Marti)