In a large bowl, combine the water, sugar, and yeast. Let the mixture rest for about 5 minutes, or until the yeast begins to bloom.

Pour half of the oil into the yeast mixture. Set the remaining oil aside.

Add the flour and salt to the yeast mixture. Stir to combine.

Add the dough to a greased bowl and cover. Let rise for 1 hour in a warm place, or until the dough has doubled in size.

Preheat the oven to 450°F.

Pour the remaining oil into a 9×11-inch baking pan. Use a pastry brush to coat the bottom of the pan.

Transfer the risen dough to the prepared baking pan, spreading it out to the edges.

In a medium bowl, combine the melted butter, brown sugar, cinnamon, and vanilla. Stir until fully combined.

Fold in the diced apples until everything is evenly coated.

Top the dough with ⅓ of the apple mixture. Gently fold the edges of the dough toward the center over the filling.

Flip the dough over so the seam side is down. Gently press and stretch the dough back out toward the edges of the pan.

Use your fingertips to create deep dimples all over the surface of the dough.

Spread the remaining apple mixture evenly over the top of the dough.

Bake for 28-30 minutes, or until the bread is golden brown and the apples are bubbly and caramelized.

In a small bowl, whisk together the confectioners' sugar and milk, 1 tablespoon at a time, until you reach the desired consistency.

Drizzle the glaze over the warm focaccia before serving.