Adjust rack to top position and preheat oven to 425 degrees. Wash and dry all produce. Trim, peel, and cut carrots on a diagonal into 1-inchthick pieces. Halve and peel onion; cut into 1/2-inch-thick wedges. Mince a few wedges until you have 1 TBSP (2 TBSP for 4 servings). Toss carrots and onion wedges on a baking sheet with a large drizzle of oil, salt, and pepper. Roast until browned and tender, 25-30 minutes.

Meanwhile, dice potatoes into 1/2-inch pieces. (TIP: For a smoother texture, peel potatoes first.) Place in a medium pot with enough salted water to cover by 2 inches. Bring to a boil and cook until tender, 15-20 minutes. Reserve 1/2 cup potato cooking liquid, then drain and return potatoes to pot. Keep covered until ready to mash.

Pat pork dry with paper towels; prick all over with a fork or knife. Season with salt and pepper. Heat a drizzle of oil in a large pan over medium-high heat. Add pork and cook until browned and cooked through, 4-6 minutes per side. Turn off heat; transfer to a plate. Wipe out pan. Meanwhile, pick rosemary leaves from stems; finely chop leaves until you have 2 tsp (3 tsp for 4 servings).

Heat a drizzle of oil in pan used for pork over medium heat. Add minced onion and chopped rosemary. Cook, stirring often, until onion is softened, 2-3 minutes. Add demi-glace and 1/4 cup water (1/3 cup for 4 servings). Bring to a simmer and cook until slightly thickened, 3-4 minutes. Turn off heat and stir in 1 TBSP butter (2 TBSP for 4) until melted.

Return pot with potatoes to medium-low heat. Add sour cream and 1 TBSP butter (2 TBSP for 4 servings). Mash until smooth, adding splashes of reserved potato cooking liquid as needed. Season generously with salt and pepper.

Divide pork, mashed potatoes, and roasted veggies between plates. Spoon sauce over pork.