1 small red onion, very thinly sliced and separated into rings
4 cloves garlic, cut into thin slivers
In a large bowl, combine 1½ cups of the flour, the yeast and salt. Add the warm water and the 2 tablespoons olive oil. Beat with an electric mixer on low to medium speed for 30 seconds, scraping side of bowl constantly. Beat on high speed for 3 minutes. Using a wooden spoon, stir in cornmeal and as much of the remaining flour as you can.
Turn out dough onto a lightly floured surface. Knead in enough of the remaining flour to make a moderately soft dough that is smooth and elastic (3 to 5 minutes total). Shape dough into a ball. Place dough in a lightly greased bowl, turning once to grease surface.
Cover and let dough rise in a warm place until double in size (45 to 60 minutes). Punch down dough; let rest for 10 minutes. Grease a 15x10x1-inch baking pan. Place dough in prepared baking pan. Gently pull and stretch dough in the baking pan into a 15x8-inch rectangle, being careful not to overwork dough.
Lightly coat dough with cooking spray. Cover loosely with plastic wrap; let dough rise in a warm place until nearly double in size (about 30 minutes).
Preheat oven to 450°F. Arrange tomato slices and artichoke quarters on a double thickness of paper towels. Let stand for 15 minutes. Change paper towels as necessary so all of the excess liquid is absorbed from tomatoes and artichokes. Using your fingers, press deep indentations in the dough 1½ to 2 inches apart. Brush dough with the 1 tablespoon olive oil. Sprinkle with rosemary. Arrange tomato slices, artichoke quarters, onion rings and garlic slivers evenly on top of dough.
Bake about 25 minutes or until golden brown. Transfer to a wire rack to cool. Cut into 12 rectangles. Serve warm or at room temperature.
To make ahead: Prepare dough as directed through Step 2. Cover and chill dough in the refrigerator for 16 to 24 hours. Punch down dough and continue as directed in Step 3, except increase the rising time in Step 4 to about 45 minutes.