Mix the flour, yeast, and salt with a spoon in the bowl of a stand mixer. Fit the mixer with the dough hook. Weigh 12 oz. (1-1/2 cups) of lukewarm water (when you dip your finger in, it should feel neither hot nor cold) and add it to the flour mixture. Mix on medium-low speed for 1 minute. With a rubber spatula, scrape the sides and bottom of the bowl and the dough hook; the dough will be very sticky. Mix, scraping down the bowl and hook every 2 minutes, until the dough looks smooth and pulls away from the sides of the bowl, about 6 minutes more.
Remove the dough hook and scrape any dough clinging to the hook into the bowl. Using a plastic bowl scraper, scrape all the dough out of the bowl onto a lightly floured work surface. Lightly flour the dough and your fingers.
Tip: To get dough off your fingers, just rub them together with a little flour over the sink or garbage can. Remove the dough from the work surface, using the bowl scraper to loosen it if necessary, and put it seam side down in a medium bowl. Cover with a linen or other flat-weave towel and let the dough rest in a draft-free spot until roughly doubled in size, 1 to 2 hours.
Line a large (17-1/2 x 13-inch) rimmed baking sheet with a linen or other flat-weave towel and generously flour the entire surface of the towel.
Using the bowl scraper, scrape the dough out of the bowl onto a lightly floured work surface, smooth top side down. Fold one side of the dough into the middle and press down firmly along the length of the seam. Fold in the opposite side and press again firmly along the length of the seam, forming a rectangle. Turn the dough over so the smooth side is up. With the bowl scraper, cut the dough into 6 equal pieces by making one lengthwise and two crosswise cuts. Weigh them; they should each be about 4-3/4 oz. Equal them out by cutting a bit off the heavier pieces and tucking it under the lighter pieces.
Make a line of flour on your work surface to dredge the baguettes. Working with one piece of dough at a time, put it smooth side down on a lightly floured work surface.
Tip: Want to see this in action? Check out our video on how to shape baguettes. Fold a long edge of the dough into the center, pressing firmly with your fingertips along the seam all the way down to the work surface, folding with one hand and pressing with the other, working from one end to the other. Fold the other long edge into the center in the same way.
Continue to fold and press alternate edges until the baguette is 11 to 12 inches long, 5 to 6 folds.
Dredge the smooth side of the dough (the seam is on top) in the line of flour.
Set the baguette floured side up (seam down) on the towel and make a little fold in the towel to separate it from the next baguette.
Repeat with the remaining dough, setting the baguettes on the towel with a fold separating them. Cover with a linen or other flat-weave towel and let sit until roughly doubled in size, 1 to 1-1/2 hours.
While the dough sits, position oven racks in the top and bottom thirds of the oven and heat the oven to 500°F (if you have a convection oven, use it). Have ready a small spray bottle of water.
Generously sprinkle two heavy-duty rimmed baking sheets (with or without sides) with semolina. When the baguettes are ready, carefully transfer them to the baking sheets with your hands, arranging 3 lengthwise per sheet. With a thin, very sharp knife, make 4 to 5 slashes on a sharp diagonal, 1/8 to 1/4 inch deep, on the tops of each baguette.
Open the oven and quickly spray about 10 squirts of water into the bottom and sides of the oven to make steam. Put the baguettes in the oven, spray again into the bottom and sides of the oven, and quickly close the door to trap the steam. Reduce the oven temperature to 475°F. Bake the baguettes for 6 minutes and then quickly turn them over on their baking sheets. Swap the sheets’ positions and bake for another 5 minutes.
Remove the baguettes from their baking sheets and put them scored side up directly on the oven racks (if making the baguettes ahead, don’t return them to the oven; see the box below). Bake until the baguettes are dark golden brown, about 5 minutes more. Transfer to a rack to cool.