Place a foil-lined baking sheet on top rack (2 foil-lined sheets, on top and middle racks, for 4 servings); preheat oven to 425 degrees. Wash and dry all produce. Trim and thinly slice scallions, separating whites from greens. Trim and halve Brussels sprouts lengthwise. Halve, peel, and mince shallot until you have 3 TBSP.

Dice potatoes into 1/2-inch pieces; place in a large pot with enough salted water to cover by 2 inches. Bring to a boil and cook until tender, 15-20 minutes. Reserve 1/2 cup potato cooking liquid, then drain. Melt 1 TBSP plain butter in empty pot over medium heat. Add scallion whites; cook until softened, 1 minute. Return potatoes to pot and mash until smooth. Season with salt and pepper. Keep covered off heat until ready to serve.

Meanwhile, carefully toss Brussels sprouts on one side of prepared baking sheet with a drizzle of oil, salt, and pepper; arrange cut sides down. Add bacon to empty side; season with salt and pepper. (For 4, divide between 2 sheets; roast Brussels sprouts on top rack and bacon on middle rack.) Roast for 10 minutes, then remove from oven. Sprinkle bacon with brown sugar. Return to oven until bacon is crispy and Brussels sprouts are tender, 10-15 minutes more.

Meanwhile, pat chicken 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 chicken and cook until browned and cooked through, 3-5 minutes per side. During the last 2 minutes of cooking, reduce heat to low and add garlic herb butter. Spoon over chicken until coated. Turn off heat. Leaving butter in pan, transfer chicken to a cutting board.

Add minced shallot to pan with reserved garlic herb butter. Cook over low heat until softened, 1-2 minutes. Stir in demi-glace, any resting juices from chicken, and 1/4 cup water. Simmer until slightly thickened, 1-2 minutes. Turn off heat. Stir in 2 packets sour cream (4 packets for 4 servings; you’ll use the rest in the next step). Season with salt and pepper.

Slice chicken crosswise. Chop bacon into small pieces. Stir remaining sour cream into potatoes; add splashes of reserved potato cooking liquid as needed until creamy. Divide potatoes and Brussels sprouts between plates. Top potatoes with chicken and sauce. Garnish chicken with scallion greens and Brussels sprouts with bacon. Sprinkle with chili flakes to taste.