Pull them and let them dry off on top of the ground. Once dry you can store somewhere cool and dry. I suspect you'll find that not even the onions are large, just start using as soon as you want to, but don't lee ptoo long as they may rot.

My onions won't be ready until September. They'll be about the size of baseballs and mighty good.

I suspect Joan will be lucky if they are bigger than golf balls! That said ours never grew larger than a tennis ball - not sure I'd want an onion the size of a baseball!

If the onions are ripening now, they are likely to be intermediate day types, which rarely develop to full size in areas with late springs, like parts of the UK. Long day onions may be a better choice for your area, or overwintering onions, if the winters aren't too severe.
Garlic and shallots are naturally mostly short day or intermediate day. Again, if the winter is mild enough, consider planting them in the fall, to harvest the next early summer. Otherwise, there are a few long day varieties--'Zebrune' Shallot is the only one I can think of off the top of my head--and they'll wait a few extra weeks to fatten up.

Did you hear about the Vidalia Onion Act of 1986? Only 20 counties in Georgia can grow Vidalia Onions.

Yep. We can grow the same varieties here, but we can't call them "Vidalias". They're just standard short day granex onions, but the low sulfur soil in those counties make them extra mild. You can get the same effect in most parts of the Sun Belt, but you might have to grow them in raised beds with synthetic soil, and with great care in fertilizing.