Plum-Tomato Focaccia

Prepare Starter: Place warm water in a small bowl; sprinkle yeast over top and stir to dissolve. Let mixture stand until foamy -- about 5 minutes. Beat in flour until smooth. Cover and let rise in a warm place until doubled in size, about 2 hours.

Prepare Dough: In a large bowl, mix together 3 1/2 cups flour and salt. Make a well in the center and add warm water in small batches, working each addition until all the water has been absorbed and a sticky dough forms. Work the starter into the dough; it will become very sticky.

Turn dough out onto floured work surface and vigorously knead, using remaining 1/2 cup flour if necessary, until elastic and smooth -- 15 to 20 minutes. Form the dough into a ball.

Clean and oil large bowl. Place dough in oiled bowl and turn it, coating the top. Cover and let rise in a warm place until doubled in size, about 45 minutes.

Coat a large baking sheet with nonstick cooking spray; sprinkle with the cornmeal. Divide dough in half. Shape each half into a round. Place rounds on prepared baking sheet, spaced about 5 inches apart. Cover; let rise in a warm place 30 minutes.

Meanwhile, position one oven rack in the bottom of the oven and position the second rack in the middle. Place a deep roasting pan filled with 2 inches of water in the center of the bottom rack. Heat oven to 375 degrees F.

When the dough has risen, slash a diamond in the center of each loaf, using a serrated knife. Cut each tomato slice in half. Arrange the tomato wedges around the diamond shape and place two sprigs of rosemary in the center of each loaf. Place the sheet pan on the middle oven rack directly over the roasting pan. (The resulting steam will ensure a crisp crust.) Bake until golden brown and crusty, 35 to 45 minutes.

Transfer breads to a wire rack and let cool 5 minutes. Brush the tops and sides with olive oil and sprinkle with kosher salt, if desired. Allow the bread to cool completely. Cut each loaf into eight slices.