Sift the white, wholemeal and buckwheat flours into a large bowl, tipping in any bran left in the sieve. Stir in the polenta, salt, yeast and sugar.
Mix together the sunflower and pumpkin kernels and linseeds, then set aside 1 tablespoon for the topping. Stir the rest into the flour mixture.
Make a well in the dry ingredients and pour in the oil and most of the water. Work the dry ingredients into the liquid to make a soft dough, adding water as needed. Turn out onto a lightly floured work surface and knead for 10 minutes, until smooth and elastic.
Place in a large, lightly greased bowl and cover with a damp tea-towel. Leave in a warm place for 1 1/2 hours, until doubled.
Turn the dough out onto a lightly floured surface and knock it back with your knuckles, then knead firmly for a few minutes. Shape into a 20 cm round and place on a lightly greased baking tray. Cover with oiled plastic wrap and leave to rise for 20–30 minutes or until well risen and springy to the touch.
Preheat the oven to 230 degrees C. Uncover the loaf and, using a sharp knife, cut the top deeply to mark it into 8 wedges. Brush with milk and sprinkle with the reserved kernels and seeds.
Bake for 15 minutes, then reduce the oven temperature to 200 degrees C. Bake for a further 15–20 minutes or until the loaf is golden-brown and sounds hollow when removed from the tray and tapped on the base. Cool on a wire rack. This bread is best eaten on the day it is made.