In a large bowl, combine the sourdough starter, 1¼ cups of water and 3 cups of flour. Stir until well combined, about 1-2 minutes. The goal is to create a smooth dough. If the dough looks a bit “shaggy,” just add more water 1 Tbsp at a time until a smooth dough develops. Cover and let dough sit at room temperature for 4 hours.

Transfer dough (still covered) to the refrigerator for 12 hours, or overnight.

In the bowl of a countertop mixer fitted with the dough hook, combine the refrigerated dough, remaining 2 cups of flour, 2-3 Tbsp of additional water, sugar, salt, yeast and citric acid (optional). Mix on low speed for 1-2 minutes. Increase speed to medium for 3 minutes, or until dough is smooth.

Place dough in a large bowl and cover with plastic wrap. Let dough rest in a warm (85°F) location for 2-3 hours, or until almost doubled in size. (The time will depend slightly on the strength of your starter, so just keep an eye on the dough occasionally during this stage.)

Divide the dough into 2 equally-sized pieces.

Gently shape the dough into 2 round loaves. Dust two parchment-lined sheet pans with cornmeal and place loaves on pans. Cover lightly and let dough rise until very puffy, about 1½ - 2 hours.

Preheat oven to 425°F.

Brush tops of loaves with the lightly beaten egg white. Combine the poppy and sesame seeds in a small bowl. Sprinkle seeds evenly over tops of loaves. Score (i.e. make shallow slices in the top of the dough) the dough with a simple cross shape.

Bake at 425°F for 25-30 minutes, or until golden brown in color.