Step 1

Combine the starter, yeast, water and flours in the bowl of a stand mixer fitted with the paddle or dough hook attachment (both work!). Mix on low a couple of minutes until combined, scrape down the sides of the bowl, then increase to speed 3 and beat for 8 minutes. The dough should be very wet and sticky, almost batter-like, but not liquid. Add more flour or water as you knead if the dough seems overly wet or dry. Sprinkle on the salt, and beat on 3 for another five minutes. The dough should still be sticky, but should pull away from the sides of the bowl while it's mixing. Leave the dough in the bowl, cover tightly with a lid or plastic wrap, and let rise 1-2 hours until doubled or tripled in bulk.

Step 2

Position a rack in the lower third of the oven and place a baking stone on top, if you have one. Place a sacrificial metal or cast iron pan on the floor of the oven - you will put ice in it to steam the oven, and it will become rusted and nasty. (UPDATE: a reader reported that the recipe works beautifully without this step, so I'm calling it optional!) Preheat the oven to 500º.

Step 3

Line a 9x12" (1/4 sheet) pan with a sling of parchment paper (the paper should lay flat in the bottom with the long ends sticking out.) Drizzle 2 tablespoons of the olive oil all over the bottom and sides of the paper. With a plastic scraper, turn the dough over in the bowl a few times, tamping out some of the air bubbles, then blob it onto the center of the oiled parchment. Drizzle the remaining oil on top and use your fingers to dimple the dough outwards towards the sides and corners. Let the dough rise until doubled in bulk, about 1 hour, until it mostly fills in the pan, dimpling out the dough a few more times to fill in the corners. The olive oil will pool in the corners, so use a teaspoon to "baste" the top of the focaccia with that oil. Sprinkle a bit of crunchy salt over the top, and any other toppings you like, and give the dough a last dimpling.

Step 4

Fill a 1 cup measure with ice cubes. Quickly place the focaccia pan on the baking stone and toss the ice cube into the sacrificial pan on the floor of the oven. Close the door and don't open it again for the next 15 minutes.

Step 5

After 15 minutes, rotate the focaccia, then turn the oven down to 450º and bake for another 5 - 15 minutes, until golden and lovely on top. Remove to a cooling rack for 10 minutes, then lift out of the pan and cool completely before snarfing.

Step 6

Store the focaccia at room temperature in a plastic bag for up to a few days (but I doubt it will last that long!)