3 tablespoons olive oil, divided
2 large bunches scallions, trimmed and cut into 1 1/2-inch pieces
16 to 20 fresh sage leaves, coarsely chopped
4 bone-in, skin-on chicken thighs (about 2 1/4 pounds)
2 1/2 teaspoons kosher salt, divided
1/2 teaspoon freshly ground black pepper, divided
1 bunch fresh thyme
1 pound small red potatoes, halved lengthwise
Arrange a rack in the middle of the oven and heat to 375°F. Pour 2 tablespoons of the oil onto a rimmed baking sheet and brush or rub with a paper towel to coat the entire surface of the pan.
Arrange the scallions over half the baking sheet. Sprinkle the sage over the scallions.
Sprinkle the chicken on both sides with 1 teaspoon of the salt and 1/4 teaspoon of the pepper. Place the chicken skin-side up on top of the scallions and sage.
Spread out the thyme sprigs on the empty half of the baking sheet and arrange the potatoes on top of the thyme in a single layer. Sprinkle with the remaining 1 1/2 teaspoons of salt and 1/4 teaspoon of pepper, and gently toss to coat. Drizzle the remaining 1 tablespoon oil over the chicken and potatoes.
Roast until the chicken is just cooked through (an instant-read thermometer inserted into the thickest part will read 165°F), and the tip of a sharp knife pierces the potatoes easily, 30 to 35 minutes.
Remove the baking sheet from the oven and increase the oven temperature to broil. Cover the potatoes with aluminum foil. Return to the oven and broil until the chicken skin just browns and crisps, being careful not to let it burn, 1 1/2 to 2 1/2 minutes. Serve immediately.
Storage: Leftovers can be stored in an airtight container in the refrigerator for up to 4 days.