Add onions and 1 tbsp butter to a grill basket lined with aluminum foil. Preheat BBQ on high. Reduce heat to medium and place grill basket, covered with aluminum foil, on the grill. Stir occasionally. Cook for 10 minutes and remove from heat.
While the onions are cooking, place mushrooms and green peppers on a ceramic BBQ grid (or directly on the grill) and sprinkle with Italian seasoning. Cook for 10 to 12 minutes, stirring occasionally. Set aside.
Turn the grill down to medium low. Place pepperoni and bacon on a ceramic BBQ grid or grill basket and cook for 1 to 2 minutes. Add the sausages to the grill and cook for 20 minutes, turning frequently. Let cool a little and slice.
Turn the grill up to medium heat. Add the Angus Burgers and cook 7 minutes per side or until done. Add a slice of mozzarella cheese to each burger during the last minute of cooking.
To Assemble
Spread pizza on the bottom of each of the six hamburger buns.
Top each with a burger with the melted mozzarella cheese.
Spread onions and top with green pepper and mushrooms.
Add pepperoni slices, sausage slices and two strips of bacon to each burger. Finish off with the top half of the bun and serve.