Adjust rack to top position and preheat oven to 425 degrees. Wash and dry all produce. Dice sweet potatoes into 1/2-inch cubes. Zest lemon until you have 1 tsp; quarter lemon. Thinly slice chives. Trim and halve Brussels sprouts.

Place sweet potatoes in a medium pot with enough salted water to cover by 2 inches. Boil until tender, 15 minutes. Drain. Melt 1 TBSP butter in same pot over low heat. Add sweet potatoes and mash until smooth. Stir in sour cream and half the chives; season with salt and pepper.

While sweet potatoes cook, toss Brussels sprouts on a baking sheet with a large drizzle of olive oil, salt, and pepper. Roast until browned and slightly crispy, about 20 minutes.

While Brussels sprouts roast, place almonds in a medium, dry pan over medium-high heat. Cook, stirring often, until lightly browned and fragrant, 3-4 minutes. Turn off heat; transfer to a small bowl. Toss with lemon zest, salt, and pepper. Wipe out pan.

Pat steak dry with paper towels; season all over with salt and pepper. Heat a large drizzle of oil in same pan over medium-high heat. Add steak and cook to desired doneness, 4-7 minutes per side. Turn off heat; remove steak from pan and let rest 3-5 minutes. Wipe out pan.

Add demi-glace and 1/4 cup water to same pan. Bring to a boil over medium-high heat, then reduce heat to low. Whisk in 1 TBSP butter and any resting juices from steak. Season with salt and pepper. Toss Brussels sprouts with almonds and juice from 2 lemon wedges. Thinly slice steak. Divide sweet potatoes, Brussels sprouts, and steak between plates. Top steak with sauce. Sprinkle with remaining chives.