Heat oil in a large frying pan; cook onion, stirring, until softened. Add washed, drained (not dried) spinach; cook, stirring, until wilted. Remove from heat; when cool enough to handle, squeeze out excess liquid. Combine spinach and herbs in a medium bowl; season.
Preheat oven to 200°C. Line a large oven tray with baking paper.
Sift flours into a large bowl; rub in butter. Stir in half the parmesan. Make a well in the centre; stir in combined buttermilk and egg, mix to a soft, sticky dough.
Knead dough gently on a floured surface until smooth. Press dough into a 20cm x 30cm rectangle. Spread spinach mixture over dough leaving 2cm strip along one long side. Sprinkle remaining parmesan and fetta over spinach. Roll up firmly, from the other long side; place on tray.
Using a sharp knife, cut the top of dough to make eight slices, without cutting all the way through. Brush with a little extra buttermilk.
Bake damper about 40 minutes or until loaf sounds hollow when tapped. Stand damper 30 minutes before serving.
This damper is best eaten on the day it is made even better eaten warm.