In a large bowl, add flour, salt, yeast, rosemary and water. With a large wooden spoon, stir to combine. Cover the bowl with a lid or plastic wrap and leave at room temperature for 18 hours.
Preheat oven to 450 degrees. Once hot, place a large Dutch oven in the oven to preheat.
Remove the dough from the bowl and scrape out onto a floured surface. Form into a rough ball. Place on a piece of parchment paper and lift parchment paper into preheated Dutch oven.
Bake for 30 minutes covered. Remove lid and continue to bake for 10 minutes.