The trick to extra-savory veggies is cooking the chicken atop the potatoes and carrots.
Preheat oven to 375°. Rub chicken thighs evenly with 1 Tbsp. olive oil. Stir together salt, pepper, and rosemary in a small bowl. Sprinkle chicken thighs evenly with 3 tsp. salt mixture.
Stir together potatoes, carrots, and onions in a large bowl. Drizzle with remaining 1 Tbsp. olive oil, and sprinkle with remaining 1 tsp. salt mixture; toss to coat.
Spread potato mixture in an even layer in a lightly greased (with cooking spray) heavy-duty aluminum foil-lined sheet pan. Place chicken thighs 2 to 3 inches apart on potato mixture.
Bake at 375° for 55 minutes to 1 hour or until a meat thermometer inserted in thickest portion of chicken registers 170°.
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