Beef Tenderloin with Mushroom Sauce

Toss Brussels sprouts on a baking sheet with a drizzle of oil, salt, and pepper; arrange cut sides down. Roast on top rack until browned and tender, 20-25 minutes. Meanwhile, melt 1 TBSP plain butter in a medium pot over medium-high heat. Add couscous and cook until toasted, 1-2 minutes. Stir in ¾ cup water (1½ cups for 4 servings) and a big pinch of salt. Bring to a boil, then cover and reduce heat to low. Cook until couscous is tender, 6-8 minutes. Keep covered off heat until ready to serve.

While couscous cooks, 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 demi-glace and ¼ cup water (⅓ cup for 4 servings) into pan with mushroom mixture, 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 plain butter until melted. Season with salt and pepper.