Adjust rack to top position (top and middle positions for 4 servings) and preheat oven to 425 degrees. Wash and dry all produce. Trim, peel, and cut carrots on a diagonal into 1-inch-thick pieces. Dice potatoes into 1/2-inch pieces.

Toss carrots on one side of a baking sheet with drizzle of oil, salt, and pepper. Toss potatoes on empty side of same sheet with a drizzle of oil, half the thyme (you’ll use the rest later), salt, and pepper. Roast on top rack, tossing halfway through, until potatoes are golden brown and carrots are tender, 25-30 minutes. (For 4 servings, divide between 2 baking sheets; roast potatoes on top rack and carrots on middle rack.)

While veggies roast, halve, peel, and thinly slice onion. Heat a drizzle of oil in a large pan over medium-high heat. Add onion and cook, stirring occasionally, until softened, 5-7 minutes. (TIP: Add a splash of water if onion begins to brown too quickly.) Reduce heat to medium and add remaining thyme and 1/2 tsp sugar (1 tsp for 4 servings). Cook, stirring occasionally, until onion is lightly caramelized, 3-5 minutes.

Add vinegar and cook until evaporated and jammy, 1 minute. Stir in stock concentrate and 2 TBSP water (1/4 cup for 4 servings), scraping up any browned bits from bottom of pan. Bring to a simmer and cook until reduced and saucy, 2 minutes more. Turn off heat; stir in 1 TBSP butter (2 TBSP for 4) and season with salt and pepper. Transfer to a small bowl. Wipe out pan.

Meanwhile, pat chicken dry with paper towels; season all over with salt and pepper. Heat a drizzle of oil in pan used for onion over medium-high heat. Add chicken and cook until browned and cooked through, 5-7 minutes per side. In the last 1-2 minutes of cooking, evenly top chicken with caramelized onion and mozzarella; cover pan until cheese has melted.

Divide chicken, potatoes, and carrots between plates. Top chicken with any remaining caramelized onion and serve.