In the bowl of an electric mixer, combine the flour, water, starter, olive oil, yeast, and salt. Mix on low speed until combined.

Scrape down the sides of the bowl to make sure all the ingredients have been incorporated.

Use the dough hook to mix on medium speed for 5 minutes until the dough is smooth, but still sticky and quite loose.

Transfer the dough to an oiled bowl and allow to rise for 1-2 hours, until puffy and almost doubled in size.

Place the blueberries, sugar, water, and sage in a small saucepan and simmer over medium heat for 10-15 minutes, until most of the blueberries have popped and cooked down.

In a small bowl, mix the cornstarch with 1 Tablespoon of the juices from the pan until smooth, and then transfer this back to the pan and mix in.

Cook an additional 3-5 minutes until thickened.

Turn off heat, stir in lemon juice and vanilla, and allow to sit and cool to room temperature. Leave the sage leaves in until its time to use.

Coat the bottom of a 9x13 pan with melted butter.

Transfer the dough to the pan and spread out with your fingers until it covered the bottom.

Dollop half of the blueberry compote evenly over the dough.

Carefully fold each end of the dough over the blueberry compote so that the dough is 1/3 of the size, envelope style.

Carefully rotate the dough so that it fits the pan better again (some of the filling will ooze, and thats ok!).

Cover the pan and allow it to rise again for another 1-2 hours,  until mostly doubled and puffy.

Preheat the oven to 425 F with the rack in the bottom third of the oven.

Drizzle the remaining compote over the top of the dough, and the use your fingers to dimple the dough, pushing the blueberries into the dough. Move from one end of the pan to the other.

Bake for 25-30 minutes, until golden.

Allow to cool most of the way in the pan, and then remove to a wire rack. Ice while still slightly warm, but not hot.

In a food processor, blend together the granulated sugar and the sage blossoms or leaves until the sugar has turned into powdered sugar and the sage is blended well into the sugar.

In a small bowl, mix the sage sugar with the milk and vanilla together.

Drizzle over the mostly cooled focaccia. If desired, you can sprinkle some sage flowers on top of the icing.