In a large mixing bowl, combine the sourdough starter, warm water, and olive oil. Stir until the starter is mostly dissolved.

Add the bread flour and salt and mix until all of the flour is hydrated and no dry patches remain.

The dough will be extremely wet, sticky, and shaggy at this stage. Resist the temptation to add additional flour — high hydration is what gives focaccia its signature airy texture.

Cover the bowl and let rest for 20-30 minutes.

Using damp hands, perform 4-6 stretch and folds or coil folds:

Grab one side of the dough.

Stretch it upward without tearing.

Fold it over itself toward the center.

Rotate the bowl and repeat until all sides have been folded.

Cover and allow the dough to rest for 30 minutes.

Repeat another set of 4-6 stretch and folds or coil folds.

If time allows, let the dough rest another 30 minutes and perform a third set of folds. This extra round helps strengthen gluten development and creates an even lighter, airier focaccia.

Cover the bowl and allow the dough to ferment at room temperature for 4-6 hours, or until noticeably puffy and increased in volume by roughly 50%.

The dough should appear smoother, jiggly, and full of air bubbles around the edges.

Cover the bowl tightly with plastic wrap or an airtight lid and refrigerate overnight or up to 24 hours.

The long cold ferment develops flavor and makes the dough easier to handle; however, if you want to bake the same day, continue on to the next steps.

Once the dough has completed its cold ferment, prepare the filling.

In a large skillet over medium heat, melt the butter.

Cook for 5-8 minutes, stirring occasionally, until the peaches soften slightly and release their juices and the mixture becomes thick and syrupy.

Remove from the heat and stir in the vanilla extract.

Allow the filling to cool for 10-15 minutes before assembling the focaccia.

In a small bowl, combine the flour and brown sugar.

Add the cold grated butter and work it into the flour mixture using your fingertips or a fork until pea-sized crumbs form.

Place in the refrigerator until ready to use.

Pour the melted butter evenly across the bottom of a standard half sheet pan.

Gently transfer the chilled dough onto the buttered pan.

Carefully stretch the dough into a rough rectangle.

Spread half of the peach filling down the center of the dough, leaving a border around the edges.

Perform a letter-style fold directly in the pan:

Fold the right side toward the center.

Fold the bottom upward over the dough.

Fold the left side toward the center.

Fold the top downward over the dough.

Next, starting from the bottom edge, gently roll or fold the dough over itself once more while flipping the entire dough so that the seam side is underneath and the smooth side faces upward.

This creates beautiful layers of peaches throughout the finished focaccia.

Cover the pan loosely with plastic wrap.

Allow the dough to rise at room temperature for 2-5 hours, or until it has relaxed, spread throughout most of the pan, and looks extremely bubbly and airy.

The exact timing will depend on the strength of your starter and the temperature of your kitchen.

Watch the dough rather than the clock.

Properly proofed focaccia should jiggle slightly when the pan is shaken and have visible bubbles throughout the surface.

Preheat the oven to 425°F (218°C).

Lightly coat your fingertips with olive oil or melted butter and gently dimple the dough all over, pressing down until your fingers nearly reach the bottom of the pan.

Spoon the remaining peach filling evenly across the top of the dough.

Scatter the remaining diced peach over the surface.

Sprinkle the crumble topping evenly over the peaches.

Bake for 30-35 minutes or until:

The top is deeply golden brown.

The crumble is lightly caramelized.

The edges are crisp and buttery.

The internal temperature reaches approximately 205°F (96°C).

If the crumble begins browning too quickly, loosely tent the focaccia with foil during the final 10 minutes of baking.

While the focaccia cools, whisk together: powdered sugar, vanilla extract, and milk until it reaches your desired consistency.

Allow the focaccia to cool for 15-20 minutes before drizzling with the vanilla glaze.

Serve warm or at room temperature.