Well grease a baking sheet or line with parchment paper; set aside. Stir together flour, cornmeal, onion, sugar, the 1-1/2 teaspoons poppy seed, baking soda, and salt in a medium mixing bowl. Using a pastry blender, cut in butter until mixture resembles coarse crumbs. Make a well in center of flour mixture. Add milk. Using a fork, stir until dough can be gathered into a ball.
Turn dough out onto a lightly floured surface. Knead for 8 to 10 strokes or until dough is almost smooth. Divide into 3 portions. Roll each portion to a 12x9-inch rectangle on a lightly floured surface. Using a fork, prick rectangles well. Using a pastry wheel, cut each into twelve 4-1/2x2-inch rectangles. Place 1 inch apart on baking sheet.
Combine egg white and 1 tablespoon water; lightly brush over rectangles. Sprinkle with 1 tablespoon poppy seed.
Bake in a 375 degree F oven for 10 to 12 minutes or until browned and crisp. Transfer to wire racks to cool. Store in an airtight container for up to 3 days. Serve with sliced cheeses, if desired. Makes 36 crackers.