In a large mixing bowl, combine the yeast, salt, and flour and stir to thoroughly mix.
Pour the water into the flour mixture and stir until a slightly uneven dough forms using a rubber spatula. If the dough seems overly wet, add a small amount of flour.
Transfer the dough onto a floured surface. Knead for 5-10 minutes until the texture becomes smooth. You'll notice the exterior is sleek and soft when gently pressed.
Place the dough in a well-oiled bowl and cover it with a damp kitchen towel. Let it rise undisturbed until it doubles in size, which typically takes 1 to 2 hours.
Drizzle the olive oil into a 12-inch cast iron skillet or a 9x13" pan. Place the dough in the pan and coat it with olive oil. Gently stretch the dough to fit the pan.
Cover the pan with a damp kitchen towel and let the dough rest until it puffs up and fills the pan, usually, this takes 1 to 2 hours.
Once the dough is ready, lift the edges to release any air bubbles trapped beneath it.
Use your fingers to create small indentations on the surface, characteristic of the classic bumps found in focaccia.
Preheat the oven to 450F.
Sprinkle the dough with sea salt and rosemary.
Position the baking pan on the middle rack of the preheated oven.
Bake for 20-25 minutes until the top turns a deep golden brown, achieving a crispy texture.
Remove from the oven, and then place the bread on a wire rack to cool before serving.