If no buttermilk the basic batter works fine : http://classicmeggle.blogspot.com/2018/10/basic-deep-fry-batter.html
Source: Felicity how to cook to perfect:
Serves 2-4
1 large/2 med onions
268ml buttermilk
80ml milk
Veg oil
100g flour
1 tsp salt
1 tsp smoked paprika
Buttermilk and milk into 1L bowl, stir
Cut the onion into 1cm wide rings, and mix into the bowl
Leave to soak 30m to 1h
Flour, salt, smoked paprika into larger bowl, mix
Heat oil to 185C (cook at 175 to 180)
Fry in small batches until golden brown
