Whisk flour and salt together in a medium bowl. Whisk room-temperature water and starter in large bowl until smooth. Add flour mixture to water mixture and stir using a wooden spoon, scraping up dry flour from bottom of bowl until dough comes together, then knead by hand in bowl until shaggy ball forms and no dry flour remains. Cover bowl with plastic wrap and let sit at room temperature for at least 12 to 18 hours.

Lay 12 by 12-inch sheet of parchment paper on the counter and spray generously with vegetable oil spray. Transfer dough to a lightly floured counter and knead 10 to 15 times. Shape dough into a ball by pulling edges into the middle. Transfer dough, seam side down, to the center of the parchment. Pick up dough by lifting parchment edges and lower into heavy-bottomed Dutch oven. Cover with plastic wrap.

Adjust oven rack to middle position and place loaf or cake pan in the bottom of the oven. Place a pot on the middle rack and pour 3 cups of boiling water into the pan below. Close the oven door and let the dough rise until doubled in size and does not readily spring back when poked with your floured finger, 2 to 3 hours.

Remove pot and water pan from oven discard plastic from the pot. Lightly flour top of the dough and, using a razor blade or sharp knife, make one 7-inch-long, 1/2-inch-deep slit along top of the dough. Cover pot and place on middle rack in oven. Heat oven to 425 degrees. Bake bread for 30 minutes (starting timing as soon as you turn on the oven).

Remove the lid and continue to bake until loaf is deep brown and registers 210 degrees, 20 to 30 minutes longer. Carefully remove bread from pot transfer to wire rack and let cool completely before serving.