Mix together the flours, turmeric, bicarbonate of soda and sea salt in a large bowl. Use your fingers to rub in the butter cubes until only small crumbs remain. Stir in the porridge oats, spring onions and coriander seeds. Pour the kefir over and use a palette knife or round-bladed knife to quickly mix in. Tip everything out onto a lightly floured worksurface and use your hands to gently knead into a smooth-ish dough. Shape into a 20cm round loaf with a flat top, and lift onto a flour-dusted baking sheet. Use a small, sharp knife to score a deep cross into the top. Bake for 35-40 minutes or until crusty and the base of the loaf sounds hollow when tapped. Cool on a wire rack while wrapped in a tea towel to soften the crust.