I'm going to answer something without answering your last question.
For every new onion leaf is another layer of onion on the bulb.
This means the faster it grows the bigger the onion.
And I think too acidic soil will cause a hot onion.
I like the hot onions for cooking.
Too much sulfur will make them hot.
When you see the first one start to bolt pull them all as they will keep longer.
Leave some for seed.
Worth