In a small bowl, combine the active dry yeast with lukewarm water. Allow it to sit for about 5-10 minutes until it becomes frothy.
In a large mixing bowl, whisk together the all-purpose flour, whole wheat flour, and salt.
Once the yeast mixture is frothy, pour it into the bowl with the dry ingredients. Add the finely chopped rosemary and mix until a rough dough forms.
Turn the dough out onto a lightly floured surface. Knead for about 8-10 minutes, or until smooth and elastic.
Form the dough into a ball and place it in a greased bowl. Cover with a kitchen towel and let it rise for about 1 hour, or until doubled in size.
After the first rise, punch down the dough and shape it into a loaf. Place it on a baking sheet lined with parchment paper.
Cover the loaf with a kitchen towel again and let it rise for another 30-45 minutes.
Preheat your oven to 375 degrees F (190 degrees C).
Once the dough has risen, sprinkle the top with flaky sea salt and make a few shallow slashes on top.
Bake for 25-30 minutes, or until the crust is golden brown and sounds hollow when tapped.
Remove from the oven and let cool on a wire rack. Slice and serve.