Mix starter and water: Add sourdough starter and water to a large bowl and stir until combined.
Add flours and salt: Mix in strong white flour, whole wheat flour, and salt until a rough dough forms.
Autolyse: Mix with a wooden spoon or dough whisk until shaggy, cover, and rest 45–60 minutes.
Pre-fold: Bring edges of the dough to the center, flip it upside down, cover, and rest 30 minutes.
Stretch and fold: Over the next 2 hours, perform stretch-and-folds every 30 minutes. Wet hands and surface slightly, lift one side, fold it over like an envelope, rotate ¼ turn, and repeat 3 more times. Keep dough covered between folds.
Bulk ferment: Transfer dough to a loaf pan or bowl, cover, and let rise in a warm spot until puffed and increased by 30–50%.
Shape the dough: Turn dough onto a lightly wet surface and stretch into a rough rectangle roughly as wide as your your loaf pan is long.
Roll and tuck: Roll the dough from the shorter side into a log, then tuck the ends underneath. Place in a greased and floured loaf pan.
Second rise: Loosely cover with plastic and let rise until the dough increases by 50–70%, nearly reaching the top of the pan.
Bake: Preheat oven to 180°C / 350°F. Bake 25–30 minutes, until golden brown and hollow-sounding when tapped.
Cool: Let the bread cool in the pan 10 minutes, then transfer to a cooling rack to finish cooling before slicing.