Preheat the oven to 400°. Line a 9-by-5-inch loaf pan with parchment paper and butter it. Sprinkle some of the flax and sesame seeds on the bottom of the pan.
In a large bowl, whisk together all the dry ingredients.
Make a well in the middle and pour in the buttermilk.
Stir just long enough to form a dough, then turn out onto a clean and floured countertop (or pastry board)and pat into a loaf, and put into the prepared pan.
Using a sharp knife, cut a 1/4-inch-deep lengthwise slash down the middle of the loaf leaving a 1-inch margin at either end.
Brush with more buttermilk and sprinkle with flax and sesame seeds (or whatever seeds you're using).
Bake at 400 for 45-50 minutes, or until the top and bottom of the bread both sound hollow when tapped.
Cool on a rack for about 15 minutes before slicing.