Feed your sourdough starter 6–12 hours before mixing the dough so it is bubbly and active.

Melt the butter in a small saucepan over medium heat. Add the onion, garlic, and salt and sauté for 5–7 minutes, until softened.

Stir in the dried basil and garlic powder and cook for 1 minute more.

Add the crushed tomatoes and sugar, then cover and simmer on low for 20 minutes.

Remove from the heat, stir in the milk, and blend until smooth with an immersion blender, if desired. Let cool to below 85°F before using.

Transfer the cooled tomato soup to a large mixing bowl. Add the active sourdough starter and whisk until combined.

Add the bread flour, parmesan cheese, and salt, mixing until no dry flour remains and a sticky dough forms.

Cover and let rest for 20 minutes.

Lightly wet your hands. Lift one side of the dough, stretch it upward, and fold it over itself. Rotate the bowl 90 degrees and repeat on all 4 sides.

Gather the dough into a loose ball and place it seam-side down in the bowl. Cover and rest for 15 minutes.

Repeat this process 3 more times, for a total of 4 rounds, resting 15 minutes between each round.

Cover the dough and let it rise until nearly doubled and bubbly, about 4–12 hours depending on your kitchen temperature and starter strength.

Turn the dough onto a lightly floured surface and gently stretch it into a rectangle.

Fold one short end toward the center, then fold the other short end over it like a letter.

Fold the remaining two sides toward the center. Flip the dough seam-side down and shape it into a tight ball.

Lightly dust a proofing basket (or a bowl lined with a tea towel) with rice flour. Place the dough seam-side up in the basket and pinch closed if needed.

Cover and refrigerate overnight, or for 12–16 hours.

Place a lidded Dutch oven in the oven and preheat to 450°F. Once the oven reaches temperature, let the Dutch oven heat for 30 minutes more.

Turn the dough onto parchment paper or a bread sling and score the top with a lame or sharp knife.

Carefully remove the hot Dutch oven, add 2 ice cubes for steam, and lower the dough inside.

Cover and bake for 50 minutes. Remove the lid and bake for 5–10 minutes more, or until the crust reaches your desired color.

Transfer the bread to a wire rack and let cool for at least 2 hours before slicing.