1) Heat 1 tbsp olive oil in a medium skillet over medium-high heat. Saute mushrooms, onions and peppers until tender, 7 to 8 minutes then season with salt and pepper. De-glaze the pan with stock.
2) Combine the sausage and sirloin with grill seasoning, garlic and parsley. Mix and form 4 patties thinner at the center and thicker at the edges for even cooking and to keep burgers from bulging. Heat cast iron or nonstick skillet with 1 tbsp olive oil over medium-high heat, cook patties 12 minutes turning once and top with mushrooms, onions and peppers and 2 slices of cheese.
3) Turn heat off and cover pan with foil to melt cheese, 2 minutes. Pile on baps and serve.