Toss the potatoes with the olive oil, salt, pepper, and garlic powder, then spread out on a rimmed baking sheet lined with parchment paper. Bake for 30 minutes until crisp, then remove from the oven and reduce heat to 350 F.
While the potatoes are cooking, add the olive oil to a large pan over medium heat.
Season the chicken liberally on both sides with the salt and pepper. Working in two batches, add the chicken to the pan with the olive oil. Sear for 5 minutes on one side, then turn over and sear for 2 minutes on the other side.
Once the chicken is finished cooking, cut into 1-inch pieces, then toss with the pizza ranch dressing, then set aside.
Toss the drained diced tomatoes with the Italian seasoning and garlic powder, then set aside.
To assemble the casserole: place the potatoes in the bottom of a round casserole dish, then top with the tomato mixture, followed by the ranch chicken mixture. Next, place the pepperonis on top of the chicken, then add the mushrooms, olives, and green bell pepper, making sure they are evenly spread out.
Bake at 350 F for 30 minutes, let cool slightly, and serve with additional pizza ranch on the side!
You can also use our 5-Ingredient ranch in this recipe!