For the sponge:
1/8 tsp. instant (rapid rise) yeast
¾ cup warm water (105-110˚ F)
6 oz. (1 cup plus 3 tbsp.) lower protein all-purpose flour, such as Gold Medal or Pillsbury
For the dough:
½ tsp. instant yeast
½ cup water (75˚ F), divided, plus 2 tsp. additional water if needed
10 oz. (2 cups) lower protein all-purpose flour, such as Gold Medal or Pillsbury
1 tsp. salt
For the glaze:
1 large egg white
1 tbsp. water
To make the sponge, combine the yeast, warm water and flour in a medium bowl. Stir with a wooden spoon until thick and smooth. Scrape down the bowl, cover with plastic wrap and cut a couple of small holes in the plastic wrap with a paring knife. Let stand at room temperature. After 4-5 hours, the sponge should be about doubled in size and have tiny bubbles on the surface. Continue to let stand at room temperature until the surface shows a slight depression in the center, about 2-3 hours longer (this never happened for me.)
To make the dough, add the yeast and 6 tablespoons of the water to the sponge. Stir briskly with a wooden spoon until the water is incorporated. Stir in the flour and continue mixing with the wooden spoon until a scrappy ball forms. Turn the dough out onto a work surface and knead by hand, adding drops of water as needed, until the dry bits are absorbed into the dough, about 2 minutes. Stretch the dough into an 8 x 6-inch rectangle. Make indentations in the surface of the dough with your fingertips; sprinkle with 1 tablespoon of the water. Fold the edges of the dough up toward the center and pinch to seal in the water. Knead lightly, about 30 seconds (the dough will feel slippery). Begin crashing the dough by flinging it against the work surface several times. (This helps the dough absorb the water.) Continue to knead and crash the dough alternately until it is soft and supple, and the surface is almost powdery smooth, about 7 minutes.
Again, stretch the dough into an 8 x 6-inch rectangle and make indentations with your fingertips. Sprinkle with the remaining 1 tablespoon of water and the salt. Fold and seal the edges once again, and knead and crash as before, about 7 minutes, until the dough feels smooth and powdery. If the dough still feels tough, knead in the additional 2 teaspoons of water.
Stretch a small piece of dough out thin (the windowpane test). If the dough does not tear and you can see light through the dough, it is adequately kneaded. (If the dough tears, knead a bit more and test again.) Form the dough into a ball, transfer to a large lightly oiled bowl and cover with plastic wrap. Let stand 30 minutes. Remove the dough, knead gently to deflate for about 10 seconds. Return to the bowl, replace the plastic wrap, and let rise until doubled, about 90 minutes.
Gently punch down the dough in the bowl, and turn it out onto a work surface. Divide the dough into two 12-ounce pieces. Working with one piece at a time and keeping the second piece covered, drag the dough to the edge of the work surface, forming the dough into a rough torpedo shape, about 6½ inches long. Repeat with the second piece of dough. Drape the dough pieces with plastic wrap and let rest 20 minutes.
Meanwhile, cover an inverted baking sheet with parchment paper. Working with one piece of dough at a time and keeping the other covered, shape the dough. Make an indentation along the length of the dough with an outstretched hand. Press the thumb of one hand along the indentation while pulling the upper edge of the dough down over the hand to enclose the thumb. Repeat this process along the length of the dough. Press the seam with your fingertips to seal closed. Roll the cylinder of dough seam-side down, rolling and stretching until it measures 15 inches long by 2½ inches wide. Place seam-side down on the prepared baking sheet. Repeat with the second piece of dough. Space the shaped dough pieces 6 inches apart on the baking sheet. Drape with a clean, dry kitchen towel and cover the sheet loosely with plastic wrap (or seal in a very large plastic bag). Refrigerate until the dough has risen moderately, 12-16 hours (no longer).
To bake the bread, place one oven rack in the lower middle position with a baking stone on the rack. Adjust the other to the lower middle position and place a small empty metal baking pan on it. Preheat the oven to 500˚ F. Remove the baking sheet with the baguettes and let stand covered at room temperature for 45 minutes. Remove the plastic wrap and towel and let stand an additional 15 minutes. Meanwhile, bring 1 cup of water to simmer in a small saucepan on the stovetop. Make the glaze by beating the egg white and water together.
With a single-edge razor blade or very sharp knife, make five ¼-inch deep diagonal slashes on each baguette. Brush with the glaze and mist with water in a spray bottle. Bring the baking stone out of the oven and line up the edge with that of the baking sheet. Quickly slide the parchment paper with the baguettes off of the baking sheet and onto the hot baking stone. Pour the simmering water into the baking pan on the bottom oven rack (be careful to avoid the steam!) Bake, rotating the baking stone after 10 minutes, until the surface is a deep golden and an instant-read thermometer inserted into the center reads 205-210˚ F, about 5 minutes longer. Transfer to a wire rack and cool 30 minutes.