Wash and dry all produce. Preheat oven to 425 degrees. Trim Brussels sprouts, then halve through stem ends. Cut sweet potato into 1/2-inch cubes. Cut fingerling potatoes in half lengthwise. Mince chives.

Toss sweet and fingerling potatoes with a large drizzle of oil on one side of a baking sheet. Season with salt and pepper. Toss Brussels sprouts with a drizzle of oil on other side of sheet. Season with salt and pepper and arrange cut-side down. Roast in oven until lightly browned, about 20 minutes.

Place pine nuts in a large pan over medium-high heat. Toast, tossing frequently, until fragrant and golden, 2-4 minutes. Remove from pan and set aside. Wipe out pan.

Heat a drizzle of oil in same pan over medium-high heat. Pat steak dry with a paper towel. Season all over with salt, pepper, and steak spice. Add to pan and cook to desired doneness, 2-5 minutes per side. Remove steak from pan and set aside to rest. Wipe out pan. Slice steak against the grain after it has rested a few minutes.

After Brussels sprouts and potatoes have roasted 20 minutes, remove sheet from oven. Drizzle Brussels sprouts with balsamic vinegar. Return to oven and continue roasting until vinegar is thickened and syrupy, about 5 minutes more. Toss Brussels sprouts with pine nuts once done roasting.

Add demi-glace and 1/2 cup water to pan used for steak over medium-high heat. Stir to combine. Bring to a boil, then lower heat and let simmer. Stir in 1 TBSP butter and chives. Season with salt and pepper, then remove from heat. Divide steak, potatoes, and Brussels sprouts between plates. Drizzle sauce over steak and serve.