Heat the milk and butter to warm (110 - 110 degrees) in microwave-safe bowl on 50% power.
Pour the warm milk and butter into a large bowl, and stir in the yeast and brown sugar until the yeast has dissolved. Allow the mixture to sit and get foamy (approximately 5 minutes).
Using the dough hook on a stand mixer, or a wooden spoon, stir in the all-purpose flour, whole wheat flour, and salt *Note - If using a wooden spoon, this will take some elbow grease and after a while, you may want to switch to using your hands.
Continue mixing until the dough comes together into a ball. Turn the dough out onto a lightly floured surface and hand-knead until the dough comes smooth and elastic (about 4-5 minutes).
Place the dough into a bowl that has been sprayed with olive oil spray and also top the dough with a light spray. Cover the bowl with plastic wrap and allow the dough to rise until double in bulk in a warm draft-free place (about 1 hour).
Punch the risen down and turn out onto a very lightly floured surface.
Roll the dough out to an approximate 10"x12" rectangle. Spread the Dijon mustard out onto the rolled dough.
Roll the dough tightly widthwise and place the rolled dough, seam side down, in a loaf pan that has been sprayed with olive oil.
Allow the loaf to rise until doubled in bulk in a warm, draft-free place (approximately 1 hour).
Bring the baking soda and ½ cup water to a rolling boil on the stove top. Brush the baking soda and water solution over the entire top of the loaf and allow to rise again for an additional 30 minutes.
In a small bowl, whisk together the egg and 1 teaspoon water. Brush the top of the loaf with the egg wash and sprinkle evenly with sesame seeds, if desired.
Use a very sharp paring knife to cut 3 slashes (approximately ½" deep) on the top of the loaf.
Preheat oven to 400-degrees.
Bake the pretzel bread for 20 minutes at 400 degrees and then decrease the oven temperature to 350 degrees and bake for an additional 10 minutes, or until the bread is deeply golden in color and sounds hollow when tapped.
Remove and allow the bread to cool on a wire race for 10 minutes. Loosen the bread by running a knife around the sides of the loaf pan and remove the bread and allow to cool completely on the wire rack until ready to serve. *Note - the bread slices best when completely cool.