Preheat a pizza stone near the center of oven to 450º F. Place a metal broiler pan on the bottom oven rack.
Dust the surface of the dough with flour. Cut off about a 1/2 -pound (227g; orange-size) piece of dough. Cover the remaining dough and refrigerate for baking loaves within 14 days.
Dust the piece with more flour and quickly shape it into a ball by stretching the surface of the dough around to the bottom on all four sides, rotating the ball a quarter-turn as you go. Shaping should take no more than 20 to 40 seconds. Cover; let dough rest for 15 minutes.
Elongate ball into an oval shape. Take one long edge, fold over and pinch into center. Pull over other edge to center and pinch together to create a seam. Turn seam-side down. Elongate dough into a rope by rolling it on the work surface with your hands and gently stretching until it is about 16 inches long. You can taper the ends by rolling them against the counter with your hands. Use enough flour while working to prevent sticking. Transfer to a sheet of parchment paper.
Cover loosely with plastic and allow to rest for 40 minutes.
Lightly brush the surface of the dough with the egg white wash.
Using a serrated knife or bread lame, slash the dough with three 1/2-inch-deep diagonal cuts along the length of the baguette.
Slide the parchment paper with loaf directly onto the hot stone. Pour 1 cup of hot water into the broiler tray. Bake for about 25 minutes, until richly browned and firm.
Allow to cool on a rack before eating.