In a large mixing bowl, combine the ground Italian sausage, ground beef, Italian breadcrumbs, egg, Italian seasoning, and kosher salt. Mix thoroughly until all ingredients are well incorporated, ensuring the seasoning is evenly distributed throughout the meat.

Divide the mixture into six equal portions and shape each into a patty, pressing them firmly to maintain their structure while cooking. Set aside.

Preheat the grill to medium heat, approximately 350-375°F, ensuring the grates are clean and lightly oiled to prevent sticking.

Meanwhile, heat olive oil in a skillet over medium heat, either on the stovetop or directly on the grill. Add the sliced green bell pepper and onion, seasoning with a pinch of salt. Sauté until they soften and the edges caramelize slightly, releasing their natural sweetness. Transfer to a plate and set aside.

In the same skillet, add the sliced cremini mushrooms and cook for a few minutes, stirring occasionally, until they become tender and slightly golden. Remove from heat.

Place the burger patties onto the grill and cook for approximately 4 minutes on one side. Flip them carefully, then top each patty with a slice of mozzarella cheese. Allow the burgers to cook for another 4 minutes, giving the cheese enough time to melt beautifully over the meat.

While the burgers finish cooking, lightly toast the hamburger buns on the grill for extra texture and warmth.

To assemble, place a grilled burger patty on the bottom half of each toasted bun. Spoon a generous amount of pizza sauce over the top, followed by four slices of pepperoni. Pile on the sautéed bell peppers, onions, and mushrooms, allowing the flavors to layer perfectly. Garnish with fresh basil leaves for an aromatic finish.

Drizzle with additional pizza sauce if desired, then cap each burger with the top half of the bun. Serve immediately while warm, and enjoy the ultimate fusion of pizza and burgers in every bite.