In a large bowl, combine the flour, yeast, salt, lemon, and rosemary. Add the water and stir until blended. It will be very sticky. Cover the bowl with plastic wrap and let rest in a warm spot (75° is ideal) for 12 to 24 hours, until the surface is dotted with bubbles.
Place the dough on a lightly floured surface then sprinkle with a bit more flour. Turn the dough over onto itself once or twice. The dough will be very wet and sticky - that's ok - just don't knead the dough. Cover loosely and let rest for 15 minutes.
Turn the dough out onto a lightly-floured work surface. Lightly flour your hands and gently shape the dough into a ball without kneading it. Sprinkle cornmeal onto a flour sack type kitchen towel and place the dough, seam side down, onto the towel. Dust the top of the dough with more cornmeal. Cover with another towel and let rise until the dough is more than double in size, about 2 hours.
30 minutes before the dough is ready, heat the oven to 450°F and place a 4-quart covered Dutch oven in the oven as it heats.
Use oven mitts to carefully remove the pot from the oven. Turn the dough over into the pot so that it’s seam-side up. If necessary, shake the pan a bit to evenly distribute the dough. Use a sharp knife to cut three shallow slits on top of the dough. Cover and bake for 30 minutes, then uncover and bake 15 to 30 minutes more, until golden brown on top.
Cool on a wire rack for 10 minutes before removing from the pot.