Preheat the oven to 230°C and oil a baking sheet.
Sift flour and bicarbonate of soda into a bowl and mix with salt.
Create a well, add buttermilk, and mix with your hand to form a soft dough.
Roll dough to 35x20cm and place on the baking tray.
Brush with oil, sprinkle with salt, and bake at 230°C for 10 minutes.
Reduce heat to 200°C and bake for 8-12 minutes until golden.
Cool bread on a wire rack and cut into squares.
Melt butter in a saucepan and coat potatoes and onions.
Cover with baking paper and sweat vegetables for 10 minutes.
Add celeriac and stock, then simmer for 10 minutes.
Blend soup while adding milk or cream for consistency.
Serve hot topped with hazelnuts, parsley, and bread.