In a large bowl, combine water, active sourdough starter, salt, sugar, cinnamon, and nutmeg (if using). Mix until combined. Add the flour and mix until no dry flour remains. The dough will be sticky and shaggy.

Cover the bowl with a damp towel or plastic wrap and let rest for 1 hour at room temperature.

Perform one set of stretch and folds to build strength. Wet your hands, grab one side of the dough, stretch it up, and fold it over itself. Rotate the bowl 90 degrees and repeat 4-6 times total until you’ve worked around the entire dough ball.

Cover the bowl and let the dough proof at room temperature for 8-12 hours, or until at least doubled in size.

In a large saucepan over medium heat, combine diced apples, brown sugar, lemon juice, vanilla, cinnamon, nutmeg, ginger, salt, and butter. Cook, with a lid on, stirring occasionally, until the apples soften, about 8-10 minutes.

In a small bowl, whisk together cornstarch with 2 tablespoons of water until smooth. Pour this into the apple mixture and stir constantly for 1-2 minutes until thickened. Remove from heat and let cool until warm but still pourable.

Grease a 9x13 inch baking pan generously with butter or cooking spray.

In a shallow bowl or plate, whisk together granulated sugar, cornstarch, and cinnamon. Set aside for dipping.

Dump dough out onto a greased surface. Pinch a golf ball size chunk of dough off. Working with one ball at a time, dip it into the warm apple pie filling, coating generously on all sides. Immediately roll in the cinnamon sugar mixture to coat completely. Place in the prepared pan.

Cover loosely with plastic wrap or a damp towel and let rise for 1-2 hours at room temperature, or until puffy and the balls have grown together.

After the second rise, pour or spoon any remaining apple pie filling over the top of the proofed focaccia.

Preheat oven to 425°F (220°C). Bake for 35-40 minutes, or until deep golden brown on top. If the top is browning too quickly, tent with foil. The internal temperature should reach about 200-205°F. Cool in the pan for 10 minutes.

In a microwave-safe bowl, combine granulated sugar and water for the caramel. Microwave on high for 1 minute, then check. Continue microwaving in 30-second intervals until it turns medium amber.

Carefully stir in warmed heavy cream, butter, vanilla, and salt. Let cool for 5-10 minutes until just warm to the touch.

Whisk in powdered sugar until smooth. The glaze should be pourable but have some body to it.

Drizzle the caramel glaze generously over the warm focaccia. Let the glaze set for a few minutes before serving.