Set oven to 200°C.
Fry the onion and garlic in a tablespoon of the oil until soft and golden. Cool.
Mix the flour, salt and baking soda together. Make a well in the middle and add the buttermilk and a tablespoon of the olive oil.
Mix to a dough. Place on a floured surface and roll out to fit rectangular, roughly 20cm x 30cm, lightly oiled baking tray.
Slice the potato as thinly as you can – on the wide bit of box grater works well.
Scatter the onion mixture over the top of the bread. Arrange the thin potato slices on top. Drizzle over the remaining oil. Scatter the cheddar evenly on top.
Bake for 25–30 minutes or until golden and bubbling on top.
Serve straight away.