Mix 1/2 cup active sourdough starter with 1 cup lukewarm water until well incorporated.
Add 3 cups flour, 2 tablespoons tallow, 1 tablespoon olive oil, and 2 teaspoons salt to the starter mixture. Mix until a shaggy dough forms.
Allow the dough to rest covered for 1 hour at room temperature.
Perform 3 sets of stretch and folds, 30 minutes apart. (Please see notes above if you are unsure how to do stretch and folds).
Cover the dough with a moist tea towel and let it ferment on the counter for 10-12 hours, or until doubled in size.
Turn the dough out onto a floured surface and shape. Then, place it in a floured banneton for a second rise.
Let the dough rise for another 2-3 hours on the countertop.
Chill the dough in the refrigerator for 1-2 hours to firm up for scoring.
Place a Dutch oven inside and preheat to 500 degrees for 1 hour.
Remove the dough from the refrigerator, transfer to parchment paper, dust with flour, and score.
Place the dough into the preheated Dutch oven, cover, and bake at 500 degrees for 30 minutes.
Uncover, reduce the temperature to 475 degrees, and bake for an additional 15-20 minutes, or until golden brown.
Allow the bread to cool on a wire rack before slicing.