Preheat oven to 400°F.

Sift together dark rye flour, whole wheat pastry flour, all-purpose flour, baking soda, and salt.

Stir in raisins (if using).

Make a well in the middle of the flour mixture and pour in the buttermilk.

Stir gently until everything comes together into a dough.

Turn the dough out onto a lightly floured surface.

Knead the dough for about 30 seconds until it forms a ball.

Slightly flatten the dough ball.

Lightly flour a baking sheet and place the dough on it.

Slice 4 deep slashes across the top of the dough (don't cut all the way through).

Lightly brush the top with buttermilk and dust with flour.

Bake for 30 minutes on the middle/lower rack of the oven.

Move the baking sheet up a rack and bake for another 20 minutes.

Cool on a wire rack before slicing and serving.