it could be a brassica, can we see the leaves and/or the whole plant?

Radish

Definitely looks brassica-ish to me, which means that those little pods at the end, along with the flowers, are quite edible.

Taste it! Great in stir fries or salads, whether it's a radish or some other brassica. Check under the soil to see if it has made an edible root as well.

By the definition of a weed being a plant out of place, it's a weed if it doesn't belong there.

Michael357, so agree with you) Fighting "weeds" - dill, calendula and alpine strawberries all over my garden now) . Finally, decided do not "let it grow"))

Looks like radish blossoms. I agree with Elisa, the very young pods are tender, and will add a little zing to salads. If you intend to try the pods, you can - to a point - regulate the degree of spiciness. Keeping the plants well watered will make the pods milder, as will cool conditions. Hot, dry conditions will intensify the flavor.

Leaves

leaves resemble radish, yes.

I had a very similar plant, I thought I had planted stevia and this came in it's spot. Being a newbie, I could not recognize and let it grow to flowering stage, today, I picked it and there was a small white radish at the bottom, but it had become all hard. The flowers were identical to the picture, so my vote is it is a radish.

Leaves also look arugula-like, but the flowers look purplish, and arugula, in my experience, is white or yellow depending on the type.

Have you tasted it yet? Checked under the soil for a bulb?
Those tests will tell you if it's arugula, radish, or some other unidentified brassica.

We are enjoying the mystery, but go ahead and look under ground to see if it's a radish!

If you have a smart or I phone, there's an app that will get back to you within 24 hours with a plant ID. I think it was called garden compos. They aren't 100%, but can be helpful! Nancy

There's Name that Plant Forum right here on GW which will probably get you an answer within an hour or so. I think it is probably a Radish too. But whatever it is it has bolted and is of no further use as a veggie except to eat the seed pods if you want to. So you might as well pull it up.