Adjust rack to middle position and preheat oven to 425 degrees. Wash and dry produce.

Peel potatoes if desired, then dice into ½-inch pieces. Trim, peel, and cut carrots on a diagonal into ½-inch-thick pieces. Trim and thinly slice scallions, separating whites from greens.

Place potatoes in a medium pot with enough salted water to cover by 2 inches. Bring to a boil and cook until tender, 10-12 minutes.

Return potatoes to pot and mash with sour cream and 1 TBSP butter (2 TBSP for 4) until smooth, adding splashes of reserved potato cooking liquid as needed. Season with salt and pepper.

Keep covered off heat until ready to serve.

While potatoes cook, toss carrots on a baking sheet with a drizzle of oil, salt, and pepper. Roast on middle rack until browned and tender, 20-25 minutes.

While carrots roast, pat chicken* dry with paper towels; season all over with salt and pepper.

Heat a drizzle of oil in a large pan over medium-high heat. Add chicken; cook until browned and cooked through, 5-7 minutes per side.

Turn off heat; transfer chicken to a cutting board. Wipe out pan.

In pan used for chicken, melt 1 TBSP butter (2 TBSP for 4 servings) over medium heat. Add scallion whites, garlic powder, and half the thyme (all for 4); cook, stirring, until fragrant, 30 seconds.

Add vinegar to pan. (TIP: Push the pan away from yourself when adding vinegar.) Once bubbles subside, stir in scallion greens, cream sauce base, stock concentrate, and 1 TBSP water (2 TBSP for 4). Cook, stirring, until thickened, 30-60 seconds.

Thinly slice chicken crosswise.

Divide chicken, mashed potatoes, and carrots between plates. Top chicken with sauce and serve.