In a large mixing bowl, combine the active sourdough starter and warm water. Whisk until the starter is mostly dissolved.

Add the bread flour and fine sea salt. Mix with a sturdy spoon or your hands until a shaggy dough forms and no dry flour remains. Cover the bowl with plastic wrap or a damp towel and let rest for 30 minutes (autolyse).

After the autolyse, drizzle 1 tablespoon of extra virgin olive oil over the dough. Using wet hands, perform the first set of stretch and folds directly in the bowl. Reach under one side of the dough, pull it up, and fold it over itself towards the center. Rotate the bowl a quarter turn and repeat 3-4 times until the dough tightens slightly. Cover and let rest for 30-45 minutes.

Repeat the stretch and fold process 3 more times, with 30-45 minute rest periods between each set. After the final set of stretch and folds, the dough should be smoother, more elastic, and developed.

While the dough is undergoing its bulk fermentation, prepare the apple filling. Peel, core, and dice the Granny Smith apples into small 1/4 to 1/2 inch pieces.

In a small saucepan over medium heat, melt the unsalted butter. Add the diced apples, light brown sugar, ground cinnamon, ground nutmeg, and lemon juice. Cook for 5-7 minutes, stirring occasionally, until the apples start to soften slightly but still hold their shape.

Sprinkle in the all-purpose flour (or cornstarch) and stir well to coat the apples and thicken any juices. Remove from heat and let cool completely to room temperature before adding to the dough.

Once the apple filling is completely cool, gently fold it into the sourdough dough. Try to distribute the apples evenly throughout the dough without deflating it too much. This is best done after the last stretch and fold.

Cover the bowl and let the dough continue its bulk fermentation at room temperature (around 70-75°F / 21-24°C) for another 2-3 hours, or until it has increased in volume by about 20-30% and shows signs of activity (small bubbles on the surface). The exact time will depend on your starter's strength and room temperature.

Generously grease a 9x13 inch baking pan with 2-3 tablespoons of extra virgin olive oil. You may also line it with parchment paper for easier removal.

Gently transfer the dough into the prepared pan. Avoid punching down the dough.

Using oiled hands, gently dimple the dough all over with your fingertips, pushing down to create characteristic focaccia indentations. Be careful not to tear the dough or push all the way through. Ensure some olive oil pools in the dimples.

Cover the pan loosely with plastic wrap or a large plastic bag and let it proof at room temperature for another 2-3 hours, or until it looks visibly puffy, airy, and has increased in volume significantly (it should spring back slowly when gently poked).

Preheat your oven to 400°F (200°C) with a rack in the middle position, at least 30 minutes before baking.

Remove the plastic wrap from the focaccia. Bake for 25-35 minutes, or until the top is golden brown and the bottom is cooked through. If desired, you can carefully remove it from the pan (if using parchment) and bake directly on the oven rack for the last 5 minutes for a crispier bottom.

Transfer the focaccia to a wire rack to cool completely in the pan for at least 30 minutes, or remove it to cool directly on a rack.

In a small bowl, whisk together the sifted powdered sugar, milk, vanilla extract, and a pinch of salt until smooth and pourable, adjusting milk as needed for desired consistency.

Once the focaccia has cooled to just warm or room temperature, drizzle the vanilla glaze generously over the top.

Slice and serve warm or at room temperature. Enjoy!