Pour water, olive oil, yeast, and salt into a lidded plastic food-grade container. Mix with a wooden spoon for a few seconds.

Add flour and continue mixing with wooden spoon  You might have to wet your hands a bit to incorporate the last bit of flour. (You can also use a stand mixer with a paddle attachment). Don’t knead the dough; it isn’t necessary. When there are no dry patches in the dough, you have finished mixing. The dough should be loose enough to conform to the shape of the container.

Cover with a non-airtight lid. Allow the dough to rise for approximately two hours at room temperature. When the dough flattens on top, refrigerate for at least four hours, and up to 14 days. The dough will collapse in the container once it is refrigerated. (That’s normal for this dough.) You can make the focaccia anytime over the next 14 days. The dough will develop sourdough characteristics as it sits.

To bake the focaccia, preheat the oven to 425 degrees Fahrenheit. Place an empty broiler pan on the bottom rack of the oven. Grease a baking sheet with olive oil.

With greased or floured hands, gently cut off a grapefruit-sized piece of dough from the dough in the container. Carefully form dough into a ball. Place onto baking sheet and flatten with your hands into a 1/2- to 3/4-inch-thick round.

Sauté onion slices in two tablespoons olive oil until soft but not browned. Spread onions sparingly over surface of the dough, leaving a one-inch border. Sprinkle with rosemary and a bit of coarse salt. Drizzle with remaining teaspoon of oil.

Allow the focaccia to rest and rise for 20 minutes.

After the focaccia has rested, place baking sheet on a rack near the center of the oven. Pour one cup of hot water into the broiler tray and close the oven door. Bake for about 25 minutes, or until crust is medium brown.