Combine the yeast, sugar, and the warm water in a large bowl until the yeast is dissolved and foamy, about 5 to 10 minutes.
In a large mixing bowl, combine the cornmeal and salt with the boiling water. Beat vigorously with a wooden spoon to blend. Add the melted butter and mix to incorporate. Now, pour the yeast mixture into the cornmeal mixture, stirring to combine. Gradually add the flour, mixing well after each addition. Add a couple of tablespoons of water if the dough feels dry and doesn't come together easily. Continue to mix until a dough forms into a ball. Turn the dough out onto a lightly floured surface and knead, adding just enough flour to keep the dough from sticking, until smooth and elastic, about 5 to 10 minutes.
Grease a large bowl with oil and transfer the dough to the bowl, brush the top with more oil so it is completely coated. Cover the dough with a kitchen towel and let it rise in a warm place until doubled in bulk, about 11/2 hours.
Turn the dough out onto the floured surface and knead briefly, 3 or 4 times, to punch out the air. Gather the dough into a ball and shape it into a round loaf. Cover the dough with a kitchen towel and let rise again until doubled in bulk, about 1 hour.
Preheat the oven to 400 degrees F. and place a pizza stone in the oven to heat up.
Put the dough round on the pizza stone and brush the top with oil. Sprinkle the top of the bread with coarse salt. Bake in middle of the oven for 40 to 50 minutes until the bread is golden brown and bottom sounds hollow when tapped. *To recreate the steam effect from authentic Portuguese brick ovens, spray the bread and oven walls with cold water every 10 minutes. Transfer the bread to a wire rack to cool.