Heat the oven to 425°F. Set a rack in the middle position and place a large rimmed sheet pan in the oven while it heats.

Put the halved baby potatoes and red onion wedges in a large bowl.

Add 1 1/2 tablespoons olive oil, half of the ranch seasoning mix, 1/4 teaspoon kosher salt, and a few grinds of black pepper to the bowl. Toss until the potatoes and onion are evenly coated.

Carefully remove the hot sheet pan from the oven. Spread the potatoes and onion on the pan in one layer, with as many potatoes as possible cut side down.

Return the sheet pan to the oven and roast for 15 minutes.

While the potatoes roast, put the chicken thighs in the same bowl. Add 1 tablespoon olive oil, the remaining ranch seasoning mix, and a few grinds of black pepper, then rub the mixture over both sides of the chicken.

Put the green beans in a clean bowl.

Add the remaining 1/2 tablespoon olive oil, the remaining 1/4 teaspoon kosher salt, and a few grinds of black pepper to the green beans. Toss to coat.

After the potatoes have roasted for 15 minutes, pull the sheet pan from the oven. Turn the potatoes with a spatula so the browned sides face up.

Push the potatoes and onion toward the edges of the sheet pan. Put the chicken thighs skin side up in the open spaces on the pan.

Return the sheet pan to the oven. Roast for 20 minutes.

Pull the sheet pan from the oven again. Scatter the green beans around the chicken and between the potatoes.