Preheat the oven to 400°F (200°C).
Prepare the casserole dish: Lightly spray a 9x13-inch baking dish with nonstick cooking spray.
Mix the seasonings: In a small bowl, whisk together the olive oil and all seasonings including the ranch.
Assemble the ingredients: Add the potatoes, onion, chicken, (and vegetables, if using) to the baking dish. Toss everything together in the herbed olive oil until evenly coated.
Cover the baking dish tightly with foil and bake for 30 minutes.
Remove the foil and bake for an additional 15 minutes, or until the chicken is fully cooked and the potatoes are tender.
Sprinkle the shredded cheddar cheese and cooked bacon evenly over the top. Return to the oven, uncovered, and bake for another 5–10 minutes, until the cheese is melted.
Garnish and serve: Top with sliced scallions and serve with a dollop of sour cream, if desired.