1-1/3 cup cooked grated potato (put it through ricer or use leftover mashed potato)
1 1/2 teaspoon dried yeast
2 tablespoons olive oil (for the dough, more for greasing the pan and adding to the top of the dough)
1-1/4 teaspoons salt
3-1/2 cups all-purpose flour total
1 cup warm water total
Make a starter with 1-1/2 teaspoons of yeast, 1/2 cup flour and 1/2 cup warm water between 105 F and 115 F.
Mix together in the bowl used to mix the dough. Cover with plastic wrap. Let it sit for about 20 minutes. It will foam and rise a bit.
After 20 minutes add 1/2 cup warm water, 2 tablespoons olive oil, grated potato and 1 1/4 teaspoons salt.
Mix the dough in standing mixer with the dough hook. You can also mix with a wooden spoon or by hand. 5 minutes stand mixer, 40 seconds food processor. Add remaining 3 cups flour. If kneading it by hand it will be very loose, wet and shaggy.
Drizzle the dough with a little olive oil. Cover it with a towel, warm to rise double in size.
Spread it out on a cookie sheet greased with about ¼ cup olive oil. Dimple it with oiled fingers to spread it into corners. If desired, push black olives into the top.
Allow to rise until puffy. Drizzle olive oil over the top, sprinkle with kosher salt and some crushed rosemary, fresh or dried.
Pre-heat the oven to 425F. Bake the focaccia until it is golden brown 23-25 minutes.
I try to get it out of the pan quickly and onto a cooling rack so the crust stays crispy.