Place 2 baking sheets lined with foil or parchment paper inside your oven; preheat to 425 degrees. Wash and dry all produce. Trim and halve Brussels sprouts. Trim and thinly slice scallions, separating whites from greens. Halve, peel, and mince shallot until you have 3 TBSP. Dice sweet potatoes into 1/2-inch cubes.

Place sweet potatoes in a large pot with enough water to cover by 1 inch. Boil until tender, about 15 minutes. Drain. Heat 1 TBSP regular butter in same pot. Add scallion whites, salt, and pepper. Cook for 1 minute, then add sweet potatoes and mash until smooth. Keep covered until ready to serve.

Meanwhile, toss Brussels sprouts with a drizzle of oil, salt, and pepper on one preheated sheet; arrange bacon on other preheated sheet. Sprinkle sprouts and bacon with salt, pepper, and half the brown sugar. Roast until browned and crispy, 20-25 minutes.

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, 5-8 minutes per side. During 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 plate.

Add minced shallot to pan with reserved garlic herb butter. Cook on low until softened, 1-2 minutes. Stir in demi-glace and 1/4 cup water. Simmer until slightly thickened, 1-2 minutes. Turn off heat. Stir in 4 packets sour cream and any chicken resting juices. Season with salt and pepper.

Slice chicken crosswise. Crumble or chop bacon into small pieces. Stir remaining brown sugar and remaining sour cream into mashed sweet potatoes. Divide Brussels sprouts and sweet potatoes between plates. Top potatoes with chicken and sauce. Garnish chicken with scallion greens. Garnish Brussels sprouts with bacon. Sprinkle with chili flakes (to taste).