Whisk the starter into the lukewarm water (about 80°F) until fully dissolved, then add the flour and mix. Rest 30 minutes, then squeeze in the salt, olive oil and rosemary with wet hands.

Do four sets of stretch and folds, 30 minutes apart, and bulk ferment until about 50 percent bigger, 4 to 6 hours total.

Divide the dough into 2 pieces of about 440 g each and rest 10 minutes, covered.

Roll each piece into a 28-inch rope with flat hands, resting the dough if it springs back.

Twist the two ropes around each other and pinch the ends together tightly.

Form a ring on a parchment-lined baking sheet and pinch the ends together so the seam disappears. Set an oiled oven-safe bowl in the center if you like.

Proof for 1 to 2 hours at room temperature, or 8 to 12 hours in the fridge, until puffy.

Preheat the oven to 425°F (220°C). Brush with olive oil, sprinkle with flaky salt, and bake 28 to 32 minutes until deep golden and the center reads about 205°F (96°C). Cool on a wire rack about 30 minutes before slicing.