Adjust rack to top position, place a baking sheet on rack, and preheat oven to 425 degrees. Wash and dry all produce. Dice sweet potatoes into 1/2-inch pieces (no need to peel). Trim and halve Brussels sprouts. Trim and thinly slice scallions, separating whites from greens. Peel orange. Cut one half into segments; juice other half into a medium bowl.

Remove preheated baking sheet from oven. Carefully toss sweet potatoes on one half with a large drizzle of oil, half the Southwest Spice (use the rest as you like), salt, and pepper. Toss Brussels sprouts on other half with another large drizzle of oil, salt, and pepper. Roast until lightly browned, about 15 minutes.

Meanwhile, pat chicken dry with paper towels; season all over with salt and pepper. Heat a large drizzle of oil in a medium pan over medium-high heat. Add chicken and cook until browned but not yet cooked through, 3-4 minutes per side. Turn off heat; remove chicken from pan and set aside. Wipe out pan.

Once veggies have roasted 15 minutes, remove baking sheet from oven. Nestle chicken between veggies, return to oven, and roast until chicken is cooked through and veggies are tender, about 10 minutes. Transfer chicken to a plate and let rest 3-5 minutes.

Melt 1 TBSP butter in pan used to brown chicken over medium heat. Add scallion whites; cook 1 minute. Stir in orange juice, stock concentrate, 3 TBSP water, and 1 tsp sugar. Bring to a simmer and cook until thickened, 3-4 minutes. Reduce heat to low and stir in 1 TBSP butter, orange segments, and any resting juices from chicken. Season with salt and pepper.

Divide roasted veggies and chicken between plates. Top chicken with sauce. Garnish with scallion greens and chili flakes (to taste).