Approximately one hour before you plan to mix the dough (which is one hour before you anticipate that your starter will be ready to use), heat the milk in a small saucepan just until it comes to a simmer. Pour heated milk into the bowl of an electric stand mixer. Mix in poppyseeds. Allow to sit for approximately one hour.
Add the flour, sugar and starter to the milk and poppyseeds. With the dough hook of the mixer, mix until combined. It will be clumpy. Add the water in three parts, continuing to knead with the dough hook. Add the salt and continue kneading. If the dough is not lifting from the sides and bottom of the bowl, turn the mixer off and use a bowl scraper to help move it towards the center of the bowl. Add the butter in four parts, just until fully combined into the dough. Add the zest and knead in for a bit. It is ok if the butter and zest are not yet fully incorporated into the dough. Stop the mixer, cover the dough, and allow it to rest for 20 minutes.
With the dough hook, continue kneading the dough for about 3-5 minutes. The dough should just clean the sides of the bowl when the mixer is turned to its highest speed.
Remove the dough from the bowl to a lightly floured surface. Knead the dough for a minute or two until it is smooth. Shape into a ball and place in a lightly greased bowl. Cover the bowl with plastic wrap or a dish towel. Allow the dough to rest for 4 hours in the warmest part of your kitchen. The dough should almost double in size.
De-gas the dough and roll it again into a ball. Place in the bowl with a tight cover or plastic wrap on top. Refrigerate overnight, at least 10 hours.
In the morning, remove the dough from the refrigerator and allow it rest at room temperature for 1½ - 2 hours.
Lightly grease the pan and the bottom of the pan’s lid. Lightly flour your work surface and roll the dough out into a rectangle the width of the length ofyour pan. In other words, the width of the dough should be no bigger than 9 inches, and the dough should be approximately ¼ inch thick. Roll the dough up tightly like a cinnamon roll, pinching the horizantal end into the rest of the log. Place the dough into the prepared pan and secure the cover in place. Allow the dough to rest in a warm place in your kitchen until the dough just about reaches the top of the pan. This usually takes approximately 4-6 hours.
Preheat the oven to 350° F. With the cover on the pan, bake for 20 minutes. Carefully remove the cover and continue to bake for an additional 25 minutes, until the top is golden brown and the internal temperature is at least 200° F.
Allow the bread to cool in the pan for 5 minutes before releasing it. Let bread cool completely on wire rack.