Place a foil-lined baking sheet on top rack (for 4 servings, use 2 foil-lined sheets on top and middle racks); preheat oven to 425 degrees. Wash and dry produce. • Trim and thinly slice scallions, separating whites from greens. Trim and halve Brussels sprouts lengthwise. Halve, peel, and mince onion until you have 3 TBSP.
Dice potatoes into ½-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 ½ 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 servings, divide between 2 sheets; roast Brussels sprouts on top rack and bacon on middle rack.) • Roast on top rack 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 onion 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 ¼ cup water. Simmer until slightly thickened, 1-2 minutes. Turn off heat. • Stir in two packets sour cream (four 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 mashed 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.