In a large bowl, mix lukewarm milk, melted butter and sourdough starter. Stir to combine.

Add the remaining ingredients: all-purpose flour, sugar, salt and baking soda (if using), mixing until dough is formed. Dough will be very sticky at the beginning.

Cover the dough with plastic wrap or a clean damp towel and let it rest for 30 minutes, in a warm spot of your kitchen. You can also use a proofing box, or let it rest inside the oven with the oven's light on.

Transfer dough to a clean bowl, and start with the stretch and folds. Stretch and fold the dough over itself 4 times, letting it rest for 20 minutes.

Repeat stretch and fold 3 times more, always letting the dough rest for 20 minutes between each set.

After the last set of stretch and fold, dough will feel softer and be a lot easier to handle. It also won’t be as sticky.

Cover the dough, and let it bulk rise in a warm spot. My dough usually doubles, or almost doubles in size. For me, it usually takes between 3 and a half to 5 hours, depending on the weather, until bulk fermentation is finished.

When done, dough will feel lighter and soft.

Lightly flour a clean counter or surface and open the dough in a rectangle with your hands or a rolling pin. Do it gently, and don’t stress about deflating the dough: sandwich bread should not have big holes, so it’s ok to lose the big air bubbles.

Roll the dough over itself loosely, and pinch it closed.

In a 9x5 inch loaf pan lined with parchment paper, place shaped dough, pinched ends facing down. Cover lightly to avoid skin formation.

Let it rise until light and puffy: it usually takes from 1 and a half to 3 hours.

Preheat oven to 350ºF.

Bakeloaf for 40 to 45 minutes, or until golden brown on top. When ready, the crust will look golden brown, and when tapped, it will have a hollow sound. If using a thermometer, the internal temperature will be between 180-190ºF.

Remove from loaf pan, and let it cool in a cooling rack for at least 1 hour before slicing.