Poisonous Plant if Consumed in Large Quantities – What is it

This herb is not that well known but adds a real kick to certain recipes

Do you know what this is?  It is apparently quite poisonous … It is very acidic and adds a very unusual kick to certain meals.  It is well known in Europe and mainly used in soups.  We were told to fry up some onions, this herb and some creme-fraiche:  We used it on fried (pre-bolied potatoes) and it was FANTASTIC!  – What is it called?