In a medium mixing bowl, whisk together flours, salt, and baking powder. Cut in butter or lard starting with a pastry cutter, then switching to your fingers. The dough should look like a coarse meal once all of the butter is cut in.
Stir in sourdough starter, water, and honey, mixing with a wooden spoon or your hands until the dough comes together. Knead a few times to make a cohesive dough, adding water a tiny bit at a time if needed to bring it together.
Cover dough bowl and allow to sour for 4 to 18 hours, depending on preference. The shorter time will produce a faint sourdough taste while the longer time will ferment the grains further and produce a more pronounced sourdough flavor.
When ready to bake, preheat oven to 400°F. Line a large baking sheet with a sheet of parchment paper. Press dough onto the lined pan to create a rectangle, and lightly dust with additional sorghum or millet flour. Top dough with another sheet of parchment paper and roll the dough out until it almost covers the entire sheet, or until the dough is very thin.
Peel away top piece of parchment paper.
Cut dough into 1-inch squares with a sharp knife or pizza cutter. Score the dough all over with a fork and sprinkle the top of the crackers lightly with sea salt.
Bake in preheated oven for approximately 15 minutes or until golden brown. Allow to cool before serving.