Adjust rack to top position (top and middle positions for 4 servings) and preheat oven to 425 degrees. Wash and dry all produce. Trim and halve Brussels sprouts lengthwise. Halve potatoes lengthwise. Trim and thinly slice mushrooms. Peel and mince or grate garlic.

Toss Brussels sprouts on one side of a baking sheet with a drizzle of oil, salt, and pepper. Toss potatoes on empty side with a drizzle of oil, salt, and pepper. Arrange potatoes cut sides down. Roast on top rack until everything is browned and tender, 20-25 minutes. (For 4 servings, divide between 2 sheets; roast potatoes on top rack and Brussels sprouts on middle rack.)

While veggies roast, pat beef dry with paper towels; season all over with salt and pepper. Heat a large drizzle of oil in a large pan over medium-high heat. Add beef and cook to desired doneness, 4-7 minutes per side. Remove from pan and set aside to rest.

Heat a drizzle of oil in same pan over medium-high heat. Add mushrooms and cook, stirring, until softened and lightly browned, 3-4 minutes. Add garlic and cook until fragrant, 1 minute.

Stir in demi-glace and 1/4 cup water (1/3 cup for 4 servings), scraping up any browned bits from bottom of pan. Bring to a simmer, then reduce heat to medium and cook until thickened, 2-3 minutes. Turn off heat; stir in sour cream and 1 TBSP butter until melted. Season with salt and pepper.

Divide Brussels sprouts, potatoes, and beef between plates. Top beef with sauce and serve.