Mix sourdough starter, flour, and water in a small bowl until well combined. Transfer to a clean jar and let rise until doubled in size and bubbly (this can be done the night before in cooler weather, or in the morning in warmer conditions). 20 g starter, 40 g all-purpose flour, 40 g water
In the bowl of a stand mixer fitted with a dough hook attachment, combine milk, sugar, doubled sourdough starter, egg, flour, and salt. Mix into a thick and slightly sticky dough. 180 g whole milk, 30 g granulated sugar, All the sourdough starter, 350 g all-purpose flour, 1 large egg, 1 teaspoon salt
Add in the softened butter and mix again for around 10-15 minutes on medium speed until the dough is silky and strong and passes the window pane test. 55 g unsalted butter
Shape into a ball and place in a container with a lid, or a bowl and cover it.
Let the dough rise in a warm spot (ideally around 25°C/77°F) for anywhere from 2-5 hours, depending on temperature, until it's bulked out by around 30-40%. You can create a warm and humid spot by placing the dough into a turned-off oven alongside a dish of boiled water. Replace the water as needed when it cools down.
After this rise, ensure the dough is covered tightly and refrigerate overnight.
The following day, remove from the fridge and let it come to room temperature for 15 minutes. Meanwhile, mix together the softened butter, herbs, garlic, and salt for the filling. 85 g unsalted butter, 2 Tablespoons fresh parsley, 2 cloves garlic, 1/4 teaspoon salt
Line a 9x5 inch loaf pan with parchment paper.
Pull the dough onto a lightly floured surface and form it into a 12-inch (30cm) long log and cut it into 12 equal 1-inch pieces with a bench scraper or knife. If you want them even, weigh the whole dough first and divide it by 12.
Shape each piece into a little ball. Roll each ball into a circle around 4 ½ inches (11cm) in diameter.
Spread each piece with garlic butter, leaving a little border around. Add a sprinkle of cheese, then fold the dough circle in half. 50 g grated cheese
Place the half circle into the loaf pan with the edges facing up. Continue with the remaining dough and filling, stacking the layers of dough. They will need to be squished in.
Cover the pan and let the dough rise again in a warm spot until doubled in size, around 4-6 hours depending on temperature.
Preheat the oven to 180°C (350°F). Whisk together egg and water for the egg wash and use a brush to brush this all over the top of the bread. 1 egg + 1 Tablespoon water
Bake for around 40-50 minutes until deep golden brown. If the top of the berad is browning too fast, loosely cover with a sheet of aluminum foil. After baking, brush the hot bread with melted butter and sprinkle a bit of flaky salt. Melted butter, Flaky salt
Leftover sourdough pull apart bread can be stored in an airtight container for up to 3 days. After day 1, reheat it in the microwave before eating to soften it up, as the enriched bread will stiffen on the following days.