In a large mixing bowl, whisk together the bread flour, whole-wheat flour, yeast, and kosher salt until well combined.
In a separate bowl, mix the warm water, melted butter (reserve 1 tablespoon for later), and honey until the honey is fully dissolved.
Pour the wet mixture into the dry ingredients and stir until just combined. Don’t worry about getting it perfectly smooth; a few lumps are okay!
Cover the bowl with a kitchen towel or plastic wrap and let it rise in a warm place for about 1 to 2 hours, or until it has doubled in size.
Once risen, lightly flour your work surface and turn the dough out onto it. Gently shape it into a rectangle, then roll it up tightly to form a loaf.
Place the shaped dough into a greased loaf pan. Cover it again with a kitchen towel and let it rise for another 30 to 45 minutes, or until it has puffed up.
Preheat your oven to 350°F (175°C) while the dough is rising so it’s ready when your bread is.
Brush the top of the loaf with the beaten egg mixture, using the reserved tablespoon of melted butter for a golden finish.
Bake the bread in the preheated oven for 30 to 35 minutes, or until the top is golden brown and the loaf sounds hollow when tapped.
Remove the bread from the oven and let it cool in the pan for about 10 minutes. Then, turn it out onto a wire rack to cool completely before slicing.