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

Trim, peel, and cut carrots on a diagonal into 1-inch-thick pieces. Trim and thinly slice mushrooms (skip if your mushrooms are pre-sliced!). Trim and thinly slice scallions, separating whites from greens.

Toss carrots on a baking sheet with a drizzle of oil, salt, and pepper. Roast on top rack until tender, 25-30 minutes.

Meanwhile, dice potatoes into ½-inch pieces (peel first for a smoother texture). Place in a large pot with enough salted water to cover by 2 inches. Bring to a boil and cook until tender, 15-20 minutes.

Reserve ½ cup potato cooking liquid, then drain and return potatoes to pot. Keep covered off heat until ready to mash.

While potatoes cook, pat chicken* dry with paper towels and season all over with salt and pepper.

Heat a drizzle of oil in a large pan over medium-high heat. Add chicken and cook until browned and cooked through, 3-5 minutes per side.

Transfer to a plate.

Heat pot with drained potatoes over low heat; add half the sour cream (you’ll use the rest later) and 1 TBSP butter (2 TBSP for 4 servings). Mash potatoes until smooth and creamy, adding splashes of reserved potato cooking liquid as needed. Season with salt and pepper.

Heat a drizzle of oil in pan used for chicken over medium-high heat. Add mushrooms and scallion whites; season with salt and pepper. Cook, stirring, until softened, 2-4 minutes.

Stir in stock concentrate and ¼ cup water (⅓ cup for 4 servings). Bring to a boil, then reduce to a low simmer. Cook until slightly thickened, 1-2 minutes.

Turn off heat; stir in remaining sour cream and 1 TBSP butter. Season with salt and pepper.

Heat pan with sauce over medium low; add chicken and spoon sauce over top. Evenly top chicken with Monterey Jack. Cover pan until cheese melts, 1-2 minutes.

Divide chicken, carrots, and mashed potatoes between plates. Spoon remaining sauce over chicken and potatoes. Garnish with scallion greens and serve.