Step 1Place flour, baking powder, 1/8 teaspoon salt, onion, cheese, millet and butter in food processor fitted with metal blade. Mix until cheese and butter are coarsely chopped. Drizzle buttermilk evenly over mixture. Pulse until dough starts to clump together. Do not pulse longer than necessary. Dough will be very wet.
Step 2Transfer to well-floured board. Sprinkle top of dough with flour. Fold dough over and back onto itself until smooth, 3 to 5 times, adding flour as needed to keep it from sticking to board. Press dough to 1/2-inch thickness.
Step 3Cut with 2 1/2-inch round cutter. Transfer biscuits to greased baking sheet. Mix egg and 1/2 teaspoon salt with fork until lightly blended. Lightly brush tops of biscuits with egg glaze.
Step 4Bake at 400 degrees until golden, about 16 minutes. Transfer to wire rack. Cool at least 15 minutes before serving. (Can also be completely cooled, then frozen in an airtight plastic bag for 1 month. Let come to room temperature before reheating. Serve warm. Biscuits reheat best in microwave.)