I find Stuttgarter to be very good long storage onions. I’m still using some of them.
But even better is this red onion variety.
Rossa di Milano
https://www.johnnyseeds.com/vegetables/onions/full-size-onions/rossa-di-milano-organic-onion-seed-3454G.html
Plus they are open pollinated so they seeds will breed true and you can save them.
My plan for onions is to every year let a batch go to seed.
This year, some of the Stuttgarters. Next year I’ll do the Milano’s.
I know we are told that onion seeds are only viable for one year, but the year before last I found some onion seeds that were two years old, this was their second year. So I sowed the whole bunch of them, and boy, did I ever get a lot of plants!
I don’t know how much the germination was affected, but I sure got all the plants I needed.
The beauty of saving your own seeds is that you can have far more than you need and of course, they are free!
At 37.9° latitude, that's cutting it tight but worth a try.
Rain - 1-1/2" yesterday and 9" total for July
* Sun Sugar -- definitely less prone to splitting than Sungold -- decent taste
* Black Krim -- splitter
* Sonic -- medium -- decent tasting -- not major splitter
* Mountain Fresh -- decent sized globe shape and not a splitter
* Mortgage Lifter -- big with the typical beefsteak ribbed shape but black in the crannies
Looking at pics on the web, some Mortgage Lifters have the crooks and crannies but not black in them. Some are globe shaped. People tend to post pics of their best produce so it's hard to judge by them.
This is a brandywine but my biggest Mortgage Lifter is headed this way. Edible but not purty.

Any beefsteak I've grown, which is not many, has been black in the crannies.
Scapify AI calls it blossom end rot and one of the black crannies does turn into a black stripe across the blossom end. The other four globe shaped mortgage lifter fruits have a dark spot where the blossom was but it's not mushy and doesn't look to be spreading.
Scapify says When there's a lack of calcium, or if the plant is unable to absorb enough calcium from the soil due to inconsistent watering (either too much or too little), the developing fruit can't form properly, leading to the blackened areas you're seeing.
Sounds like it's our crazy weather which is typical weather here. Will have to look for beefsteaks with blossom end rot resistance or just stay away from beefsteaks.