Stir together warm water, yeast and sourdough starter until the starter is dissolved.
Mix in whole wheat flour and let sit for 7-10 minutes to hydrate.
Sprinkle in salt and begin adding all purpose flour. Start by adding 1 cup of all purpose flour, stirring until combined. Then add additional flour a little bit at a time until you have a shaggy bread dough. (The final amount will depend on the consistency of your starter and how humid it is.)
I like to start my kneading in the bowl to contain the mess, then turn it onto the counter once the dough comes together and starts getting a little elasticity. You can use your preferred kneading method to knead the dough into a smooth ball, about 8 minutes.
Place dough in a lightly greased bowl and cover with a damp towel and let rise until doubled in size (about 1½ to 2 hours).
To shape the dough, pat it out into a rectangle. Fold it in thirds, like you would a letter. Then fold fold it in half, pinching together the seam. (Or use your favorite loaf shaping method.) Place seam side down in a greased loaf pan.
Cover with the damp towel and let rise for 1-1½ hours or until it has risen over the edge of the pan.
Brush gently with melted butter, being careful not to deflate the dough.
Bake at 450 F for 10 minutes and then at 400 F for 25-30 minutes.
When finished baking, turn onto a wire rack and cool completely before cutting.
Notes
You can use sourdough discard or unfed starter from the fridge in this recipe without any issues.