In a large bowl, combine semolina, flaxseed, bread flour, salt and yeast.
Add sourdough starter and coconut treacle. Add water and mix to combine.
Cover with plastic wrap sprayed with non stick spray and a kitchen towel. Leave until doubled in volume, approximately 2 hours.
Fold over dough from the bottom a few times and transfer to a lightly greased Dutch Oven. Cover and leave to rise again until doubled in volume, approximately 2 hours.
Preheat oven to 450 F.
Make a few slashes in the loaf with a sharp knife. Brush the top of the loaves with water and sprinkle sesame seeds on top.
Bake for 30 minutes. Uncover and bake for another 15 minutes, rotating pot half way through.
Notes
I maintain a multi grain sourdough starter that I feed with a combination of rye, whole wheat and all purpose flour.