Combine 1 cup all-purpose flour, whole wheat flour, undissolved yeast, 1 tablespoon onion, and salt in large mixer bowl. Heat milk, water, butter, honey and mustard until very warm (120° to 130°F) add to flour mixture. Beat 2 minutes at medium speed of electric mixer, scraping bowl occasionally. Add eggs and 1 cup flour; beat 2 minutes at high speed. Stir in enough remaining flour to make a soft dough. Knead on lightly floured surface until smooth and elastic, about 8 to 10 minutes. Cover; let rest 10 minutes.
Divide dough into 12 equal pieces; shape each into smooth ball. Place on greased baking sheet. Flatten balls with hand to about 2-1/2-inch rounds. Cover; let rise in warm, draft-free place until doubled in size, about 1 hour.
Brush tops with Egg Glaze; sprinkle with onion. Bake at 400°F for 15 to 20 minutes or until done. Remove from sheet tray; cool on wire rack.
Egg Glaze: Combine egg and water in a small bowl. Stir to blend.