ready in 2 h. 10 min.
In a large bowl, combine warm water and yeast, sugar and 1/2 cup of all-purpose flour. Set aside until foamy and bubbly, about 10 minutes.
Stir in the remaining flour, olive oil, salt and rosemary. Knead until smooth and stretchy, about 5 minutes. Rinse out bowl and coat with olive oil. Transfer the dough to the bowl and cover with plastic wrap. Set dough aside in a warm spot, to rise for 1 hour.
Remove dough from bowl and divide into four pieces. On a lightly floured work surface, roll out each piece into a 4 by 12-inch rectangle. Cut the dough lengthwise into 1/3-inch wide strips. Place strips 1/2-inch apart on a baking sheet lined with parchment paper or a greased baking sheet. Repeat the process with the remaining dough. Let the grissini rise until puffed (about 30 minutes). Season with sea salt, if desired. Meanwhile, preheat the oven to 400º F.
Bake in batches, on the top rack of the oven until lightly browned, about 10 minutes, rotating the baking sheet half way through, for even baking. Cool on a wire rack and continue until all batches are baked. Carefully store completely cooled grissini in a sealed plastic storage bag. Serve with your favorite prosciutto.