In a large mixing bowl, whisk together the bread flour and fine sea salt. Add the instant yeast and granulated sugar and stir to combine. Add the sourdough discard and warm water, then mix with a wooden spoon or your hand until no dry flour remains. The dough will look rough and shaggy at this stage.
Cover the bowl with plastic wrap or a damp kitchen towel and let the dough rest at room temperature for 1 to 1.5 hours, until it has roughly doubled in size. Timing varies with kitchen temperature.
Lightly flour a clean work surface and turn the dough out onto it. Using a bench scraper or your hands, gently fold the edges of the dough toward the center, working around the entire piece. Flip the dough over seam-side down and gently drag it toward you on the work surface to tighten the surface. Place the shaped dough seam-side up into a generously floured proofing basket or a floured bowl lined with a kitchen towel.
Cover the shaped dough loosely with plastic wrap or a damp towel and let it rest for 30 to 45 minutes at room temperature, until slightly puffed and soft to the touch. While the dough proofs, place the Dutch oven with its lid in the oven and preheat both to 475°F (245°C) for at least 30 minutes.
Cut a piece of parchment paper slightly larger than the base of your Dutch oven. Carefully flip the proofed dough out onto the parchment paper so the smooth side faces up. Using a bread lame or sharp knife, score the top of the dough with a single swift, decisive slash about 1/2 inch (1.25 cm) deep at a slight angle.
Carefully remove the preheated Dutch oven from the oven and remove the lid. Using the parchment paper as handles, lower the dough into the Dutch oven. Cover with the lid and bake for 20 minutes. Remove the lid and continue baking uncovered for another 20 to 25 minutes, until deep golden brown. The internal temperature should read at least 200°F (93°C).
Carefully lift the bread out of the Dutch oven using the parchment paper and place it on a wire cooling rack. Allow the bread to cool for at least 45 to 60 minutes before slicing to ensure the crumb sets properly.