In large bowl, combine yeast, 1 1/2 cups flour, and 1 1/2 teaspoons salt.
In 1 quart saucepan, heat 4 tablespoons olive oil and 1 1/3 cups water over medium heat, until very warm (120° to 130CF.).
With mixer at low speed, beat liquid into dry ingredients just until blended.
Increase speed to medium; beat 2 minutes, scraping bowl often with rubber spatula.
Add 1/2 cup flour; beat 2 minutes.
With spoon, stir in 1 1/2 cups flour to make a soft dough.
On lightly floured surface, with floured hands, knead dough about 8 minutes, working in more flour (about 1/2 cup) while kneading.
Cover dough and let rest 15 minutes.
Grease 15 1/2 by 10 1/2 jelly roll pan; sprinkle with cornmeal.
Press dough evenly into pan; cover and let rise in warm place (80° to 85°F.) until doubled, about 30 minutes.
Preheat oven to 400°F.
Press fingers into dough almost to bottom of pan, making indentations 1 inch apart.
Drizzle with 1 tablespoon olive oil.
Arrange sliced tomatoes over top; sprinkle with chopped rosemary, pepper, and remaining 1/2 teaspoon salt.
Bake focaccia in top third of oven until top is lightly browned, about 35 to 40 minutes.
Remove to wire rack; drizzle with remaining 1 tablespoon olive oil.
Cool slightly to serve warm.