Grease a medium bowl with vegetable oil or nonstick cooking spray. Grease a 9-in-by-5-in (23-cm-by-12-cm) loaf pan with nonstick cooking spray or softened butter. Line the bottom and two short sides with parchment paper and grease again.

To make the dough, in the bowl of a stand mixer fitted with the paddle attachment, combine the all-purpose and bread flours, yeast, sugar, salt, and potato starch and mix until incorporated. Add butter, milk, and egg and mix in medium-low speed until a shaggy dough forms, and all of the dry bits at the bottom of the bowl are incorporated.

Replace the paddle with the dough hook attachment. On medium to medium-high speed, knead the dough until a smooth mass forms that comes off the sides of the bowl and sticks only a bit to the bottom, if at all, 5 to 7 minutes. If the dough is very sticky, add a little extra flour. Alternatively, if it’s too dry, add a bit of milk, until it sticks just a bit.

Transfer the dough to the prepared bowl. Cover the bowl with plastic wrap, and set aside in a warm place for 1 to 1 1/2 hours, until it has practically doubled in size.

To make the filling, coarsely chop the leeks. Melt the butter with the olive oil over medium high heat in a cast-iron skillet or medium pot, like a Creuset. Once melted, add the leeks and the salt and stir with a wooden spoon. Reduce the heat to low and cover, cooking until the leeks soften, stirring occasionally, about 8 to 10 minutes. Remove the cover, increase the heat or medium-high, and continue to cook, stirring occasionally until the leeks are quite soft and lightly browned, about 5 minutes. Remove from the heat and bring to room temp.

In the dough feels too sticky to work with, lightly flour the counter before turning the dough out onto it. Using your hands, shape the dough into a rectangle, with the longest side closest to you. With a rolling pin or your hands, form a rectangle roughly 20 in by 15 in.

Melt the remaining 3 tablespoons of butter and generously brush the dough with the it. Evenly sprinkle the black pepper, cheese and leeks over the dough, pressing them in with your fingers.

Using a bench scraper or sharp knife, cut the rectangle the long way into 6 equal strips. Stack the 6 strips on top of each other and cut the layered strips the short way into 6 equal pieces. Stack the 36 pieces upright in the prepared pan, like you would slices of bread, or dominoes. The leeks and cheese may fall off as you do this, just stick them back into the dough or sprinkle them on top of the loaf.

Tightly wrap the pan in plastic wrap and refrigerate overnight. The next morning, let the bread sit at room temperature for about 30 to 45 minutes before removing the plastic and baking.

Or, if you want to bake right away, loosely cover the pan with plastic wrap and set aside in a warm place for 30 to 45 minutes, or until the bread rises to about 1 1/2 times its original size.

Preheat the oven to 375-degrees F.

Bake the bread until nicely browned, 40 to 45 minutes. Tent the loaf with aluminum foil at the 35-minute mark, to keep the top of the loaf from getting too brown. The bread is done with its internal temperature is 200-degrees F. If you do not have a thermometer, stick a paring knife in between the “flakes” to confirm that the dough is cooked through.

Transfer the pan to a wire rack and brush the top with the extra melted butter, melting more if need be. Let cool until the pan is easy to handle, remove the load, and let cool, right-side-up, on a wire rack. Serve warm.

The bread is best the day it is made, but can be stored, tightly wrapped in plastic wrap, on the counter for up to 3 days. Toasting the day-old bread is awfully nice, too.