Sirloin in a Mushroom Sauce

Toss carrots on a baking sheet with a large drizzle of oil, salt, and pepper. Roast on top rack, flipping halfway through, until tender, 25-30 minutes. Meanwhile, place potatoes in a medium pot with enough salted water to cover by 2 inches. Boil until tender, 15-20 minutes. Drain well; return potatoes to pot.

Meanwhile, heat a large drizzle of oil in a medium, preferably nonstick, pan over medium-high heat. Add onion; cook, stirring occasionally, until browned and softened, 8-10 minutes. Add 1 tsp sugar (2 tsp for 4) and a splash of water; cook until caramelized, 2-3 minutes more. Season with salt and pepper. Turn off heat; transfer to a small bowl. Wipe out pan.

Meanwhile, heat pot with potatoes over low heat. Mash with sour cream, salt, and pepper until smooth. Keep covered until ready to serve. Pat steak dry with paper towels; season all over with salt and pepper. Heat a large drizzle of oil in pan used for onion over medium-high heat. Add steak; cook until browned, 3-6 minutes per side. Transfer steak to a second baking sheet. Roast until cooked to desired doneness, 4-7 minutes. Set aside to rest.

Melt 2 TBSP butter (3 TBSP for 4 servings) in pan used for steak over medium-high heat. Add mushrooms and cook, stirring, until softened, 2-4 minutes. Add garlic and cook until fragrant, 30 seconds to 1 minute. Stir in demi-glace and ¼ cup water (⅓ cup for 4). Bring to a boil, then reduce to a low simmer. Stir in mustard. Simmer until slightly thickened, 3-4 minutes. Turn off heat; stir in 1 TBSP butter until melted.