Preheat the oven: Set to 425°F (220°C).
Line a large rimmed sheet pan with parchment for easy cleanup.
Season the potatoes: In a bowl, toss potatoes with 1 tablespoon olive oil, 1/2 teaspoon salt, 1/4 teaspoon pepper, and half the ranch seasoning. Spread on the sheet pan. Roast for 10 minutes to give them a head start.
Prep the chicken: Pat chicken dry.
In the same bowl, mix BBQ sauce, remaining ranch seasoning, smoked paprika, and garlic powder. Add chicken and toss to coat well.
Add the veggies: In the bowl, toss bell pepper, red onion, and broccoli with remaining 1 tablespoon olive oil and a pinch of salt and pepper.
Load the pan: Pull the potatoes from the oven. Push them to one side.
Add the chicken in a single layer and spread the veggies on the remaining space. Avoid crowding so everything browns.
Roast: Return the pan to the oven for 18–22 minutes, flipping the chicken once halfway. Chicken is done at 165°F internal temperature; veggies should be tender with caramelized edges.
Optional glaze: Brush a tablespoon of extra BBQ sauce on the chicken during the last 3 minutes for a glossy finish.
Finish and serve: Let rest 3–5 minutes.
Sprinkle with chopped parsley or chives. Drizzle with ranch, if you like. Serve straight from the pan.