Gather all ingredients.

Stir yeast into bread flour in a bowl. Add salt, followed by 4 teaspoons oil, and water. Stir with a spoon until a wet, sticky dough forms.

Cover and let sit at room temperature for 12 to 14 hours.

Uncover dough and transfer to an oiled work surface. Using oiled hands, press and push the dough into a rectangle, 16x12 inches in size.

Fold dough into thirds horizontally; fold vertically into thirds. Transfer smooth side up into a generously oiled pan.

Cover the pan with plastic wrap and let rest until almost doubled in size, for 1 hour.

Using oiled hands, fold once more into thirds horizontally, then into thirds vertically to develop gluten structure.

Flip dough, smooth side up, on the pan; cover with plastic wrap and let rest for 1 hour more.

Unwrap and stretch and fold dough once more in each direction using oiled hands.

Transfer, smooth-side up, into a generously oiled metal baking pan. Lightly drizzle with additional olive oil and stretch to fit the pan. Cover and let rest for 2 hours.

Preheat the oven to 425 degrees F (220 degrees C). Unwrap and poke holes in the top of the dough using oiled fingers if desired for the traditional focaccia look. Scatter over rosemary and flaky sea salt.

Bake in the center of the preheated oven until browned, about 30 minutes. Let cool in the pan for 10 minutes; transfer to a wire rack and let cool to room temperature, about 30 minutes.