One of the tricks to a chewy, evenly baked multigrain challah is a good combination of different grains and seeds. I have tested many multigrain flours over the years, and some have been more successful than others. The ones I like have a combination of white and whole wheat flours, cracked wheat, cracked rye, and whole flax. Every country has their own options, and you may have to do a bit of experimenting, but it
Yields 5 large challahs
Place the sugar, yeast, and half the warm water in the bowl of an electric mixer (strong enough to handle five pounds of flour) Let the yeast proof for about 15–20 minutes in the bowl.
Add the flours, sunflower seeds, bran, oil, eggs, and remaining water, alternating the wet and dry ingredients as you add. Add the kosher salt as a dough begins to form, and knead it at medium speed for seven to eight minutes.
Place about one teaspoon of oil in the center of a large bowl. Transfer the dough into the bowl and flip it to coat completely with the oil. Place the bowl in a large garbage bag and knot the bag loosely. Let it rise for one hour.
Take challah, and discard the challah piece according to your minhag. Form the remaining dough into your desired shapes. Combine the ingredients for the egg wash and brush it on the dough. Let the dough rise for another 45 minutes to one hour. Meanwhile, preheat the oven to 375 degrees Fahrenheit.
Bake large challahs for 40 minutes, and smaller individual challahs for about 25 minutes.