Dissolve salt and sugar in 1 1/2 cups warm water that is between 100 and 120 degrees F. (Warm water is needed to activate the yeast, but if it is too hot, it will kill the yeast. Use a candy thermometer to test the temperature.) Stir in entire contents of packet of yeast and allow the mixture to "form a head" (like on a beer), which takes about 15 minutes.
Place flour into mixing bowl and add yeast mixture. Stir to combine and then knead until the dough comes together and is a cohesive mass. Knead for a few minutes so the dough becomes elastic. Cover bowl with a clean cloth and allow to rise and rest for 30 minutes. Then punch the dough down, knead again briefly, cover the bowl with the cloth and allow to rise a second time.
For the filling:
Heat the olive oil in a saute pan and cook onion until it begins to caramelize. Add garlic and sliced pepper, and cook until they soften. Add pieces of sausage allowing them to brown. Pour off fat and stir in tomato sauce and Italian seasoning. Season with salt and pepper. Cook together for a few minutes to allow flavors to blend, then remove from heat and set aside briefly.
Preheat oven to 450 degrees F.
On a floured surface, stretch the dough into a 14-inch circle which is thicker around the edges. Transfer to a lightly greased 11-inch pie pan and shake cornmeal over the dough. Spoon the tomato/sausage mixture into the dough and sprinkle mozzarella over the top. Bake until the edges are golden brown and the pie sets up, about 10 to 15 minutes. Serve immediately.