Add the warm water to the bowl of a stand mixer.
Add the yeast. If using active (not instant) yeast, add a small pinch of sugar, stir, and wait 10-15 minutes until bubbly. If using instant yeast, continue to the next step.
Add the oil, rosemary, sugar, flour, and salt. Mix to combine, 2-3 minutes.
Run your mixer on medium for 5-7 minutes until a springy, smooth dough emerges.
Place in a lightly greased bowl in a warm place and let rise for 1-2 hours or until doubled.
Gently deflate and shape into a round. The "seam" will be on the bottom.
If you have a proofing basket, place the dough into the well-floured basket, seam side up. Cover with lightly oiled plastic wrap and let rise until just about even with the rim of the basket, 30-45 minutes.
If you don't have a proofing basket, place seam-side down on a parchment-lined baking sheet. Cover with lightly-oiled plastic wrap and let rise in a warm place 30-60 minutes.
Heat oven to 425°F (220°C)
If using the banneton, remove the plastic wrap and place a piece of parchment paper over the dough. Put a sheet pan upside-down on the parchment. Flip the whole thing and gently lift the banneton to release the dough. Let rise some more while oven is heating.
If desired, dust the top of the boule with flour and then make a couple of slashes with a lame or a sharp knife.
In the bottom of the oven put a pan of hot water.
Place the pan in the oven and bake for ~30 minutes -- turning half-way through -- until browned and sounding hollow and an instant-read thermometer put into the bread reads 190°F (90°C).