Preheat your oven to 375°F (190°C). Grease a Bundt pan or a 10-inch round baking dish with cooking spray or butter.
Heat olive oil in a large skillet over medium heat. Add the Italian sausage, breaking it into small crumbles as it cooks, until browned and no longer pink, about 5-7 minutes.
Add the diced onion and bell pepper to the sausage and cook until softened, about 4-5 minutes.
Stir in the minced garlic, Italian seasoning, and red pepper flakes (if using). Cook for another 30 seconds until fragrant. Remove from heat and allow to cool for 5-10 minutes.
In a large mixing bowl, combine the cooled sausage mixture, mozzarella cheese, cheddar cheese, Parmesan cheese, basil, and parsley. Mix well.
On a lightly floured surface, roll out the pizza dough into a large rectangle, approximately 18x12 inches. If using two 1-pound balls of dough, roll each into a 12x9 inch rectangle.
Cut the dough into roughly 2-inch squares using a sharp knife or pizza cutter. Don't worry about making them perfect—rustic is good here!
Taking one square of dough at a time, place about 1-2 tablespoons of the sausage and cheese mixture in the center. Fold the edges over to seal, forming a small ball with the filling inside. Pinch the seams to seal.
Place half of the filled dough balls into the prepared pan, seam-side down. They should be touching but not squished.
Brush these dough balls with half of the melted butter, then repeat the layering process with the remaining dough balls and finish with the rest of the butter.
Bake for 25-30 minutes until the bread is golden brown and the cheese is bubbling. If the top is browning too quickly, cover loosely with aluminum foil.
Let the bread cool in the pan for 5-10 minutes before inverting onto a serving plate. This cooling time allows the cheese to set slightly so it doesn't all run out when flipped.
Garnish with additional fresh basil and serve warm with marinara sauce for dipping.