Preheat the oven to 375 degrees F, or if using an Aga, use the baking oven.
To proof the yeast: in a measuring cup, put 1.5 cups of hot water, along with the yeast and 1 tsp of sugar. Stir briefly. Allow to sit in a warm place until foam forms on the top of the water.
In a stand-up mixer, add the 3 cups of flour. Pour in the yeast/water mixture while mixing on the lowest setting. Add in the sugar and salt. Mix with a bread paddle for a few minutes, until the dough is formed. It will be very sticky; you can add a small bit of extra flour if needed to handle it, but not much.
In a square pan, pour in the tablespoon of olive oil. Put the bread dough into this pan and roll it around in the oil so it gets coated. Don't worry too much about shaping the dough to the pan at this point. Cover with a clean tea towel and let sit somewhere warm for 20 minutes.
Once the dough is risen, gently use your fingers to stretch it into the shape of the pan. You're trying not to deflate too much of the air that has formed inside.
Poke holes into the top of the bread gently using a finger. Brush on some additional olive oil on top, and sprinkle on sea salt and rosemary.
If you have time, allow the bread to rise for another 5 minutes or so in the pan. If not, pop it into the hot oven for 20 minutes.
If the top of the bread isn't as brown as you'd like by the end, put it closer to the top of the oven for an additional 3-4 minutes.