So-called Lebanese Cress

[Richard Wright]

Can somebody help me (I'm not a botanist) identify an edible 
herb. I bought it at a nursery as "Lebanese Cress", with the 
genus name of Aethionema. This genus belongs to the 
Cruciferae, or cabbage family.

Now that it is flowering with white umbels I can see that 
the plant is a member of the Umbelliferae family, which 
includes parsley and carrot.

The plant has pinnate leaves up to 30cm long. It spreads by 
creeping runners and tastes pleasantly of a cross between 
parsnip and parsley.

I have read (with no description) of a Middle Eastern 
"cress" that belongs to the genus Sium. I wonder whether 
this is it.