The best onions I ever raised(and I'm no expert on onions) I grew in a very small bed that I could flood irrigate. I used miracid as fertilizer, but don't remember how often. My soil is very alkaline here, as is the water from the hose so miracid worked good. Onions don't like drought so don't let the soil dry out. steve

FWIW : I measure with a rain gauge and try and give most plants 1/2" of water, twice a week. It rained last weekend and we totaled 8/10 ths of an inch. Yesterday I watered about a half an inch more.

Also make sure they are mulched to retain the moisture and control weeds.

You may need more if it is windy or very dry air.

Other folks may do it differently.

HTH

Matthew It's hard to tell someone how to water and fertilize.You have to see what works for you by trial and errer.We don't know what kind of soil or weather conditions you have? Like sandy soil will need more water & fertilizer than clay soil and so on.Go easey and see what works for you.
YOPPER

If Your Garlics and onions' tops are nice green and strong, so you are doing fine job.
But if pale green then they need nitrogen, mostly.
Another thing is to plant them as shallow as you can so the buls grow almost out of soil.
I woul think that this way the bulbs can expand in volume much freely.
Then you have to water lightly and more frequently, just enough to keep the roots moist.
Heavy mulching can help to reduce drying out.

The good thing is that watering regiment is only needed for a short time at the later weeks just before harvest.
Before that it is either winter or spring with cool weather and lots of rain.

Right now, when I have time, I loosen up the soil around them and push it away, exposing part of the bulbs.
Then cover them lightly with mulch.