Put watercress or spinach in a bowl, cover with plastic wrap, and microwave 30 seconds to one minutes until bright green. Remove and chop fine.
In a large bowl, sift flour, baking powder, salt and sugar. Add butter and blend well with fingers. You may use a paddle attachment of a mixer or food processor as well. When well combined, add eggs and greens. Mix until it all comes together. Do not knead or overwork.
Flatten into a disc, wrap in plastic wrap, and set in the refrigerator or leave on the counter while you make the filling.