Ok, first remember I am in zone 5 at 5000 feet elevation. A whole lot different climate than you have I am sure.
I plant onions the first of April. I plant them in the garden. I don't start my own. The long day onions will be bulbing in late June and July.
I can get seed, plants or little bulbs to plant from the store.
Seed, planted the first of April will make bulbs but they are not usually vary large, maybe golf ball size. They do make a lot of very nice scallions.
Plants or bulbs planted early April will make a nice large bulb.
Now for you, I would suggest if you are going for large bulbs, you likely need short day onions and you would do well to plant plants or sets now, directly in the garden. They will then bulb up in the short days of April and May.
I suspect it gets too hot there in July and August for onions to do well?
If you are going to grow your own plants from seed for later planting out, I suggest starting 8 weeks before the date you want to plant them out.
By planting the seed directly in the garden, you can actually gain some time as it does set plants back some when transplanted.
At this point you would do well to just plant the long day seed directly in the garden. It should do OK. For that matter plant all your seed directly in the garden. Its too late for starting them as others have said. Try to not get them too thick, but you can later pull some for green onions and thinning and let some go for bulbs.
Gardening at 5000 feet elevation, zone 4/5 Northern Utah, Frost free from May 25 to September 8 +/-