Combine yeast, ½ cup of warm water, and sugar in a small bowl. Let rest for 5-10 minutes to activate the yeast. It should look a little bit foamy.
In the large bowl of a stand mixer, whisk together flour and salt. Add the yeast mixture. Gradually (not all at once) add remaining ¾ cup of warm water and mix (or stir by hand with a wooden spoon) until a soft dough comes together in a ball. You might not need all of the water. If the dough is too sticky, you can add a bit more flour. It shouldn’t be sticking to the sides of the bowl.
Once the dough comes together in a ball, knead with a dough hook or by hand until smooth and elastic – about 4-6 minutes. If the dough is sticking to the bowl or to your hands, add extra flour (one tablespoon at a time).
Transfer the dough to a greased bowl. Turn to coat the dough with grease on all sides. Cover and let rest in a warm place until doubled in size, about 45 minutes.
Punch the dough down to release the air.
Cut the dough into two equal portions.
Working with one piece at a time, flatten the dough slightly into a small rectangular shape. Fold one edge into the center, and seal the seam. Turn the dough around, and repeat on the other side. Repeat this process again, using your thumb and the heel of your hand to seal the seam as you go.
With the seam side down, gently roll from the center outwards, lengthening the dough into a log that’s about 12-14 inches long. Taper each end of the log slightly to create the baguette's typical "pointy" end, if you like.
Place the dough onto a cornmeal-dusted baking sheet or baguette pan. Cover with lightly greased plastic wrap; allow the loaves to rise until they're slightly puffy (but not doubled in bulk), about 25-45 minutes.
Towards the end of the rising time, place a cast iron pan (or other non-glass oven-proof pan or bowl) on the lowest rack in the oven. Place another rack in the middle of the oven. Preheat oven to 450 degrees F. Bring 1 ½ cups of water to a boil.
When your dough is puffy, remove the cover and use a baker's lame (a special curved blade), a razor blade, or a very sharp knife held at about a 45° angle, to make three to five long lengthwise slashes in each baguette.
Transfer the dough to the middle rack of the 450 degree F oven, while at the same time carefully pouring the boiling water into the cast iron pan. Quickly shut the oven door. The steam from the water will help to create a crusty exterior on the bread.
Bake for 15-20 minutes, or until the loaves are a deep golden brown. The bread should sound hollow when you tap it. Remove from the oven and cool on a wire rack.