In a large bowl, dissolve yeast in warm water. Add the sugar, salt and 3 cups flour. Beat on medium speed for 3 minutes. Stir in remaining flour to form a soft dough.
Turn onto a floured surface; knead until smooth and elastic, 6-8 minutes. Place in a greased bowl, turning once to grease the top. Cover and let rise in a warm place until doubled, about 1 hour.
Punch down dough. Turn onto a lightly floured surface; divide in half. Shape into two 12-in.-long loaves; taper ends. Place each loaf seam side down on parchment-lined baking sheets. Cover and let rise until doubled, about 30 minutes.
Preheat oven to 450°. Sprinkle loaves with additional flour. With a sharp knife, make three 1/2-in. deep slashes across top of each loaf. Bake until golden brown, 15-20 minutes. Remove from pans to wire racks to cool.