Place a cast iron dutch oven or a heavy pot in your oven with its lid. If the lid is not heatproof, protect the knob with aluminum foil. Preheat your oven with the pot in it to 425 degrees F.
In a medium mixing bowl, whisk together the flours, baking soda, salt and sugar. Add the seeds and whisk again to combine.
In a measuring cup, add the cider vinegar to the milk. Give it a stir and add the milk mixture to the dry ingredients. Using a rubber spatula, mix to combine just until there are no dry, floury spots.
Tear off a piece of parchment about 12 inches wide. Dump the dough onto the parchment. Using heavily floured hands, form the dough into a boule about 7 inches wide, re-flouring your hands, as needed. Brush the top of the loaf with just a bit of milk and sprinkle with a few pinches each of your seeds. With a sharp knife, cut an X across the top.
Once preheated, carefully remove the pot from the oven and close the oven door. Carefully remove the lid and using the parchment, move the loaf into the center of the pot. Replace the lid (keep those pot holders on!) and place the pot back into the oven.
Bake, covered, for 20 minutes. Remove the lid and continue baking for an additional 20 to 25 minutes, until golden on top.
Remove the pot from the oven and lift the loaf out using the edges of the parchment. Cool on a wire rack before slicing.
I like to keep my bread in a brown paper bag on the kitchen counter. It keeps for a few days.