The sprouting onion is probably sprouting a flowering stalk. If that’s the case all it will do is produce seed.
I think if you leave the onion in the ground after you collect the seed, it will produce small onion ‘sets’ that can be dug out, separated and planted for bulbs the next spring. If the onion is a hybrid, the seeds may not come out true and will not be useful to plant.
The potatoes, if they are just russets, should be able to be planted. You cut it into pieces, with one ‘eye’ or sprouting body, on each piece. Plant the pieces in mounds for easy harvest. Your county agricultural extension office is a great resource if you have question. If you have a land grant university in the area, they will also be a good resource.
This is all presuming you have a climate that can support growing root vegetables like these.
I live close to the Univerity of Florida. Their agricultural experiment station is just a few miles down the road from me. But January through June is potato season, so I’m a bit late for this year.