Kind of a small photo, but it looks to me like this...

Here is a link that might be useful: prostrate pigweed

First, if it is taking over the onion bed, it's a 'weed'...

Second, I don't *know* what it is. But the young plants in the picture tweaked the search-image part of my brain. In many ways, looks like young radish, but not quite.

I did some brief searching, and found that a wild radish relation is a weed problem in WI, and elsewhere.

I'm too tired and lazy to pinpoint a link [sorry....] but you might want to research "Raphanus raphanistrum". The pictures I found quick-and-easy were mostly of flowering stalks and older plants, but a couple were similar to your picture. But this may be what you have.

If it is a plant you do not want growing where you do not want it it is a "weed".

This is my first year of growing veggies so I am a rookie. The more the merrier!

The following has taken over the (fill-in-the-blank) section of my garden. Almost always an indicator that you have an unwelcome intruder.

I don't know if it is something else I planted that drifted there or if it is a weed. If rain caused seeds to "drift" they wouldn't be where you first put them. If everything else is still where it should be, you can attribute its' presence to a bird or squirrel "planting" it there, or the wind blew it in.

I have about fifteen different veggies planted but there is nothing else anywhere in the garden that looks like it. Very rare to get only 1 plant from purchased seeds. Suspicious!

If it is a weed, I would think it would be someplace else also, but like I said, I'm a newbie so I don't know. Any help would be appreciated. Weed seeds can come from much farther away than what you can view from your yard. However far a bird can fly or the wind can blow, that's the limits of weed seeds than can reach your yard.

What happens with the birds is that some will swallow whole seeds which pass through the bird undigested and grow wherever that bird was when nature called back its' nutritive loan.

There are weeds, then there are WEEDS. This one plant, if you don't want it, is a weed. If you decide to pull it up and it is really hard to do, then it grows back anyway, it is a WEED. It doesn't sound like you want a ton of these plants next spring so I would recommend cutting off all the flowers as soon as the petals are gone, or removing the whole plant at that time, so seeds don't ripen and fall all over the place.

Forgot to say that you can find your zone here. If you include it in your profile info, it will show up next to your name automatically when you say something on GardenWeb forums. This will allow people to offer climate-specific advice when applicable. People usually include the 2-letter postal code for their state, too, since a zone 6 in one part of the country can be a lot different than another zone 6 somewhere else.