Adjust rack to top position (top and bottom positions for 4 servings) and preheat oven to 425 degrees. Wash and dry all produce. Cut potatoes into 1/2-inch-thick wedges. Trim green beans, if necessary. Halve, peel, and mince shallot until you have 2 TBSP. Place peppercorns in a zip-close bag; crush with a rolling pin or a heavy-bottomed pan. Thinly slice chives.

Toss potatoes on a baking sheet with a large drizzle of oil and a large pinch of salt and pepper. Roast on top rack until lightly browned, 10 minutes (you’ll add more to the sheet then).

Once potatoes are lightly browned, remove sheet from oven. Push potatoes to one side. Carefully toss green beans on empty side with a drizzle of oil and a pinch of salt and pepper. (For 4 servings, leave potatoes roasting and add green beans to a second sheet; roast on bottom rack.) Return to oven until veggies are browned and tender, 10-15 minutes more.

While veggies roast, pat beef dry with paper towels; season all over with salt and pepper. Heat a drizzle of oil in a medium pan over medium-high heat. Add beef and cook to desired doneness, 4-7 minutes per side. Turn off heat; transfer to a plate and let rest for 5 minutes. Wash out pan.

Melt 1 TBSP butter in same pan over medium heat. Add minced shallot and cook, stirring, until slightly softened, 1 minute. Add demi-glace, 1/4 cup water (1/3 cup for 4 servings), 1/2 tsp crushed peppercorns (1 tsp for 4; be sure to measure), and any resting juices from beef. Simmer until slightly thickened, 2-3 minutes. Turn off heat. Stir in another 1 TBSP butter (2 TBSP for 4)and half the chives. Season with salt and more peppercorns if desired.

Divide potatoes, green beans, and beef between plates. Top beef with sauce. Sprinkle with remaining chives and serve.