In a small bowl, dissolve sugar in warm water. Sprinkle the yeast over the water and let it sit for 5 minutes until frothy.
In a large bowl, mix flour and salt. Add the yeast mixture and olive oil, stirring until a dough forms.
Knead on a floured surface for about 5 minutes, or until smooth. Place dough in an oiled bowl, cover with a towel, and let it rise for 1-1.5 hours or until doubled in size.
2. Make the Filling:
Heat olive oil in a pan over medium heat. Add the onion, bell pepper, and garlic, and sauté for about 5 minutes until softened.
Add the cooked sausage, crushed tomatoes, tomato paste, oregano, basil, red pepper flakes, salt, and pepper. Stir to combine and let it simmer for 15 minutes. Set aside to cool.
3. Assemble the Pizza:
Preheat your oven to 450°F (230°C).
Once the dough has risen, punch it down and roll it out into a large circle (about 12 inches).
Grease a 9-inch deep dish pizza pan or cast iron skillet. Press the dough into the pan, making sure it covers the bottom and sides.
Layer the mozzarella cheese on the bottom, followed by a generous amount of the sausage and tomato mixture.
Add another layer of mozzarella and a sprinkle of Parmesan cheese.
4. Bake:
Bake in the preheated oven for 30-35 minutes or until the crust is golden brown and the cheese is bubbly.
Let the pizza cool for a few minutes before slicing. Garnish with fresh basil leaves if desired.