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. Peel and dice potatoes into ½-inch pieces. (TIP: For speedier spud prep, skip the peeling.) Halve, peel, and thinly slice onion. Trim and thinly slice mushrooms (skip if your mushrooms are pre-sliced!). Peel and mince garlic.

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

Meanwhile, place potatoes in a medium pot with enough salted water to cover by 2 inches. Bring to a boil; cook until tender, 15-20 minutes. Drain and return potatoes to pot.

Mash potatoes with sour cream until smooth. Season with salt and pepper. Keep covered off heat until ready to serve.

While potatoes cook, heat a large drizzle of oil in a large, preferably nonstick, pan over medium-high heat. Add onion and cook, stirring occasionally, until browned and softened, 8-10 minutes.

Add 1 tsp sugar and a splash of water; cook until caramelized, stirring occasionally, 2-3 minutes more. Season with salt and pepper.

Turn off heat; transfer to a small bowl. Wipe out pan.

Pat steak* dry with paper towels; season with salt and pepper.

Heat a large drizzle of oil in same pan over medium-high heat. Add steak and cook to desired doneness, 5-7 minutes per side.

Transfer to a cutting board to rest.

Add 1 TBSP butter (2 TBSP for 4 servings) and mushrooms to same pan over medium-high heat. Cook, stirring, until browned and softened, 3-5 minutes.

Add garlic and cook, stirring, until fragrant, 30 seconds.

Stir in demi-glace and ¼ cup water (⅓ cup for 4). Bring to a boil, then reduce to a simmer.

Stir in mustard. Simmer until thickened, 2-3 minutes. Season with salt and pepper.

Slice steak against the grain.

Divide potatoes, carrots, and steak between plates. Top potatoes with caramelized onion. Top steak with mushroom sauce and serve.