Mix together 5 grams ripe sourdough starter with 55 grams water and 55 grams flour. Cover and let sit overnight at 78ºF until doubled in size, bubbly and active. Alternatively, if you have a ripe, bubbly, active sourdough starter, you can substitute 100 grams of bubbly starter if you don't want to make a levain.

Mix the Dough: To a bowl add 100 grams levain, 200 grams mashed potatoes, 275 grams water, 10 grams salt and 500 grams bread flour. Mix together with a dough whisk or spoon until a sticky dough forms. Cover and let rest for 30 minutes.Please Note: Mashed potatoes can vary in the amount of liquid they bring to the dough. You may need to add a little extra water or extra flour to account for a stickier or dry dough.

Coil Fold 1: After 30 minutes, take off the cover and perform 4-6 coil folds. The dough will be very sticky for this first set of coil folds but will strengthen over time. Wet your hands with water. Place your hands under the middle of the dough and pull up. The dough will stretch up (but should not tear) and release from the bottom of the bowl. Once the dough releases, let the dough fall back under itself. Repeat the process for both sides of dough. Then turn the container and repeat the coil fold. I find it very helpful to watch this process before attempting it. Cover and let rest for 30 minutes.

Coil Fold 2: Wet your hands. You will notice the dough is stronger than your first set of coil folds. Repeat the coil fold 4-6 times. Cover and rest for 30 minutes.

Coil Fold 3: Wet your hands. Repeat the coil fold 4-6 times. Notice the dough is getting stronger and the coil folds are easier to perform. Cover and rest for 30 minutes.

Coil Fold 4: Repeat the coil fold 4-6 times. Cover and rest for 1.5-2 hours.

Prepare the Pan: After the long bulk rest, pour 40 grams (3 Tablespoons) olive oil in a 12 inch round or 9 by 13 metal baking pan. Tip the pan around so the oil covers the entire bottom of the baking pan – add a little more oil if needed. Note, if your pan has issues with dough sticking, put some parchment paper down first. Add the oil on top of the parchment paper and the dough on top of that.

Prepare and Proof Focaccia: Turn the dough out into the pan and stretch slowly to fill the edges of the pan. Pull up gently on the underside of the dough to stretch it into place. If it doesn't want to stretch, let the dough rest for a minute and then try again. Cover the pan of dough with plastic wrap and let the dough rise for about 3-5 hours at 78-80ºF until doubled, airy and jiggly. If it doesn't feel this way, warm it up a little and give it more time to rise.Note: If you prefer to stick the dough in the refrigerator for a cold ferment before letting the dough rise, this can bring some extra flavor and color to the focaccia. Pull out of the refrigerator and let rise until doubled and bubbly before baking.

Dimple and Top: Once the dough has doubled in size and is ready to bake, preheat oven to 450ºF. Drizzle more olive oil (about 30 grams/2 Tablespoons) over the top of the dough. Spread it out with your hands so it evenly covers the dough. Take your fingers and gently dimple the dough. Start at the top and work your way down the dough until the entire focaccia is dimpled and bubbly. Top the dough with flaky sea salt and dried herbs if desired.

Bake: Bake for 25-30 minutes until bubbly, crispy and light golden brown on top. Let cool in the pan for 5-10 minutes before removing to a wire rack. Cool to room temperature before slicing. Enjoy!