Mix buttermilk, water, starter, and honey in a large bowl
Add flour and mix until just combined
Cover and let rest for 30 minutes to an hour
Add the salt and just melted butter, kneading until completely combined
Stretch and fold every 15 minutes for 1 hour
Stretch and fold every 30 minutes 2 hours
Bulk rise at room temperature for about 3 hours
Coat a loaf pan with butter
Gently shape the dough into a tight oval
Place seam side down in the pan
Cover with a lid or damp tea towel
Place in the fridge overnight