Lesser Celandine
Veronica persica

The yellow flower looks a bit like a buttercup, which is not surprising as it is in the same family.

However, Lesser Celandines have at least 8 petals, whereas buttercups always have 5.

The leaves have a very high vitamin C content, so used to be eaten as a cure for scurvy.

But if you want to try them make sure you pick them before the flowers appear, as they soon develop a poison.

I'd rather eat an orange myself.