Thlaspi arvense (Brassicaceae family) commonly known as field penny cress (θλάσπι) it is a rather humble plant that is mostly seen as a weed in gardens and in cultivated land. I saw this one yesterday in the park in my hometown and it is one of the first blooming plants for this season. Doing a little bit of research I found that its seeds can be used in making biodiesel and their leaves can be eaten raw or boiled. They should be collected before blooming otherwise they are bitter. They are usually added to salads.