In a bowl, stir together the flour, yeast, rosemary (if using), and salt. Add the beer and mix with a spatula until combined. Cover the bowl and let sit at room temperature for at least 12 hours, but ideally overnight. Preheat oven to 450° F. Place a large Dutch oven in the oven to preheat for 30 minutes. Generously dust a work surface with flour. Scrape the dough out of the bowl. It should be loose and sticky. Form the dough into a ball and place on a piece of parchment. Cover and allow to rise for 1 hour, until doubled in size. When ready, carefully lift the dough and the parchment and place in the Dutch oven. Bake, covered, for 30 minutes. Remove the lid and continue baking until the bread is a deep, golden brown, about 15-20 minutes more. Remove from the oven. Enjoy with softened butter and sea salt. YUM!