After nearly four inches of rain in 24 hours (3” of it in five hours), my onion plants are all leaning a bit - maybe 15 to 30 degrees off vertical. The plants are good sized and some appear to be bulbing.
Stand them up, or leave them be?
Leave them as they are. They’ve suffered enough already, and I think the best thing for them is to allow them to recover with as little additional handling as possible. The roots are already replacing damaged root hairs, and straightening them will interrupt that.
Can you provide some drainage?
Drainage is good. There was no standing water - but the soil is almost sandy so it’s like quicksand when wet-can’t support anything.
My corn has done it twice in the last week. Keeps standing back up fine on it’s own. I would think Onions would be fine…