If making from scratch, make the pizza dough.
Melt 4 tbsp of butter over medium heat in a large skillet or Dutch oven over medium heat. Add the onions, and cook, stirring frequently, until reduced and dark brown, about 40 minutes. Let cool and set aside.
In a skillet over medium heat, add the crumbled sausage and cook until lightly browned and cooked through, about 12 to 15 minutes. Remove excess grease, if necessary. Let cool and set aside.
Preheat your oven to 375°F.
Lightly flour your work surface and roll the dough out to an approximate 10" x 15" rectangular, about ¼-inch in thickness (uniform thickness is more important than the size of the rectangle).
Melt the remaining 2 tbsp of butter and brush it over the dough, leaving about 1 inch along the perimeters untouched.
Next, add the caramelized onions, sausage, and then mozzarella cheese.
Use your hands to roll the dough up starting on the longest side. Continue gently rolling until you've reached the end of the dough. Place the dough, seam-side down, on a baking sheet, preferably lined with parchment paper.
In a small bowl, make an egg wash by combining one lightly beaten egg with 1 tbsp of water. Brush the egg wash all over the top and sides of the roll of bread. Sprinkle the top with extra grated Parmesan cheese and oregano.
Bake in the oven for 30 minutes, or until nicely browned all over.
Slice and serve at once with warmed marinara sauce on the side (or your favorite dipping sauce).