Place 1 1/4 cups warm water (105 F – 115) in a medium bowl; sprinkle yeast over and stir to blend. Let stand 5 minutes to soften.

Whisk to dissolve yeast.

Add 1 1/4 cups all-purpose flour; whisk until smooth.

Cover bowl with plastic wrap. Let stand at room temperature until bubbles form and yeast mixture more than doubles in volume, about 45 minutes.

Whisk 1/2 cup warm water, olive oil, and rosemary in a large bowl to blend. Using a rubber spatula, mix in 2 1/2 cups semolina flour and fine-grained sea salt (dough will be very dry). Stir in yeast mixture.

Work in 3/4 cup all-purpose flour. Turn dough out onto a lightly floured surface. Knead until smooth, adding more all-purpose flour by the tablespoon if sticky.

Let rest 5 minutes.

Knead until dough springs back when pressed with thumb, about 8 minutes.

Lightly oil a large bowl. Transfer dough to bowl; turn to coat.

Cover with plastic wrap. Let rise at room temperature until doubled in volume, about 1 hour.

Transfer dough to a lightly floured surface. Flatten dough into an 18×12″ rectangle. Starting from one long side, roll tightly to form a 2 1/2″-diameter, 20″-long log. With seam side down, shape log into a ring, inserting one end into second end; smooth seam.

Line a baking sheet with parchment paper. Sprinkle sheet with additional semolina flour. Transfer dough ring to prepared sheet, reshaping as necessary to form a smooth circle.

Sprinkle with 1 1/2 Tbsp. sesame seeds, pressing lightly to adhere. Cover loosely with plastic wrap.

Let bread rise at room temperature until almost doubled in volume, about 45 minutes.

Heat oven to 400 F. Remove plastic wrap from bread. Using a sharp knife, cut a 1/4″ slit all the way around top of loaf.

Spray bread lightly with water. Sprinkle with remaining sesame seeds and flaky sea salt.

Transfer to oven. Bake bread 15 minutes, spraying lightly with water every 5 minutes.

Continue to bake without spraying until bread is golden brown and sounds hollow when tapped on bottom, about 30 minutes more.

Transfer bread to rack and cool completely.