Step 1

Mix or pulse yeast, 1/2 cup flour, and 1/2 cup warm water with an electric mixer or food processor until combined.

Step 2

Cover tightly with plastic wrap and set aside until bubbly, about 20 minutes.

Step 3

Add remaining dough ingredients and mix on low speed with paddle attachment until dough comes together. Switch to the dough hook and increase speed to medium until dough is smooth and elastic, about 5 minutes. If using a food processor, process until dough is smooth and elastic, about 40 seconds.

Step 4

Transfer dough to lightly oiled bowl, turn to coat with oil, and cover tightly with plastic wrap. Let rise in warm, draft-free area until dough is puffy and doubled in volume, about 1 hour.

Step 5

With wet hands (to prevent sticking), press dough flat into generously oiled 15 1/2-by-10 1/2-inch jelly roll pan or halve and flatten each piece of dough into 8-inch rounds on a large (at least 18" long), generously oiled baking sheet. Cover dough with lightly greased or oil-sprayed plastic wrap; let rise in warm, draft-free area until dough is puffy and doubled in volume, 45 minutes to 1 hour.

Step 6

Meanwhile, adjust oven rack to lower-middle position and heat oven to 425 degrees F. With two wet fingers, dimple risen dough.

Step 7

For the topping: Drizzle dough with oil and sprinkle evenly with rosemary and coarse salt, landing some in pools of oil.

Step 8

Bake until focaccia bottom is golden brown and crisp, 23-25 minutes. Transfer to wire rack to cool slightly.

Step 9

Cut rectangular focaccia into squares or round focaccia into wedges; serve warm. Can be stored on counter for several hours and reheated just before serving.