In a small bowl, combine warm water, yeast, and sugar. Stir gently and let sit for 5 to 10 minutes until foamy.
In a large mixing bowl, whisk together flour, cornmeal, and salt.
Pour the yeast mixture and melted butter into the flour mixture. Stir with a wooden spoon until a dough forms.
Turn the dough onto a lightly floured surface and knead for about 7 minutes until smooth and elastic.
Place the dough in a lightly oiled bowl, cover with a clean kitchen towel, and let it rise in a warm place for about 1 hour, or until doubled in size.
While the dough rises, prepare the sauce: Heat 1 tablespoon olive oil in a medium saucepan over medium heat.
Add the chopped onion and cook until translucent, about 5 minutes.
Add the minced garlic and cook for 1 minute until fragrant.
Stir in crushed tomatoes, oregano, basil, salt, and black pepper. Simmer uncovered for 25 to 30 minutes, stirring occasionally, until thickened. Remove from heat.
Preheat your oven to 425°F (220°C).
Heat the remaining 1 tablespoon olive oil in a 12-inch cast iron skillet over medium heat.
Add the Italian sausage, breaking it up with a spoon, and cook until browned and cooked through, about 7 minutes. Remove from heat and drain excess fat.
Punch down the risen dough and transfer it to the cast iron skillet. Press the dough evenly onto the bottom and up the sides of the skillet to form a thick crust.
Layer the shredded mozzarella cheese evenly over the dough in the skillet.
Spread the cooked sausage evenly over the cheese layer.
Pour the tomato sauce evenly over the sausage layer.
Sprinkle the grated Parmesan cheese evenly over the top.
Bake the pizza in the preheated oven for 35 to 40 minutes, until the crust is golden brown and the sauce is bubbly.
Remove the skillet from the oven and let the pizza cool for 10 minutes before slicing and serving.