Mix warm milk, warm water, sugar, and yeast in a small bowl and let proof for 5 minutes, until it is bubbly and smells sour. Meanwhile, in a large bowl, mix flours and flax seed, and then mix in butter with a fork until the mixture is the size of small crumbs. Add 2 beaten eggs, yeast mixture, and pickle juice, and mix together.
When mixture is combined and no more flour is visible, turn onto a lightly floured surface, and knead for 8-10 minutes, until dough is silky and springy. Try not to use too much flour, even though the dough is a little sticky, as adding more flour will make it tougher. Roll dough into a ball, (you may wish to coat with oil at this point to prevent sticking as it rises) return to bowl, and cover with plastic wrap. Let rise in a warm place about 1 hour, until doubled.
Prep chopped pickles and onions. Cover cookie sheets with parchment paper, or spray with nonstick spray. Pat down the risen dough, and divide it into 16 equal-sized pieces. Stretch each piece into a long, flat, rectangle, and sprinkle about 1 tablespoon chopped pickle and 1 tablespoon chopped onion down the length of it, and then roll it up. Form the rolled piece into a round, smooth bun shape, and set on cookie sheet. Repeat for all pieces of dough. It’s all right for them to touch on the cookie sheet, but they will cook together if they are too close. (They come apart easily.) Cover with plastic wrap coated lightly with nonstick spray, and let rise for 30-60 minutes, until puffed and springy to the touch.
Preheat oven to 400 °F. Beat remaining egg with 1 tablespoon of water, and brush mixture over finished buns. Sprinkle with sesame seeds, and any remaining pickle and onion. Bake for 15-18 minutes, until golden brown on top. Let cool, then slice in half and enjoy.
Notes: Keeps up to 2-3 days in sealed container in cool place, and even longer in fridge. Tastes TERRIFIC toasted. Recipe can be halved if desired.