Preheat the oven to 450 degrees F.
Combine the all-purpose flour, whole-wheat flour, butter and salt in a stand mixer fitted with the paddle attachment. Mix on medium-low speed until the mixture resembles a coarse meal. Add the onion, herbs and milk; mix until just combined--do not overwork the dough.
Roll out the dough into a 1/8-inch-thick rectangle on lightly floured parchment paper. Brush the top with egg white and sprinkle with the seeds and some salt.
Transfer the dough and parchment to a baking sheet and cut into 2-by-3-inch rectangles.
Bake until golden, 10 to 12 minutes.