In a large bowl or storage container with lid, whisk together flour and salt.
Pour warm water in a smaller bowl or large measuring cup and gently stir in the sugar and yeast. Let sit for 5 minutes while bubbles/foam forms. (Hot tip: do a quick search on google if not sure what this step should look like!)
Pour yeast mixture into the flour mixture and stir to combine. Making sure to get any loose flour at the bottom. Drizzle 2-3 tablespoons of olive oil over the top, cover with lid (plastic wrap or damp towel works too), and place in fridge to rise for 10-12 hours... It should triple in size!
Grease baking pan with butter and olive oil, making sure to get the sides well. Add a little olive oil to your finger tips and transfer dough to prepared baking dish. Press out into the shape of the pan, making sure the top is level and flat-ish (it doesn't have to be perfect). Cover with plastic wrap (I like to brush it with olive oil first) or a damp cloth. Let sit and rise for 2-4 hours at room temp... it should double (or a little more) in size.
Preheat oven to 425 degrees and get out your flaky salt, olive oil, and fresh rosemary. Add oil to finger tips again and press into the dough to make dimples. Sprinkle the top with olive oil, salt, and rosemary.
Bake in the oven for 20-25 minutes or until the bread is a pretty golden brown.
Remove from oven, transfer to cooling rack or cutting board and let cool before cutting. We love this as a side or sliced and half and served as the bread of a sandwich or bun of a burger! (YUUUUUMM!)