Halve and peel onion. Cut halves into ½" dice. Mince garlic. Stem and mince parsley. Pat chicken breasts dry, and season both sides with a pinch of salt and pepper.
Start the Vegetables
On one half of prepared baking sheet, toss butternut, onion, garlic, half of parsley (reserve remaining for garnish), 2 tsp. olive oil, ½ tsp. salt, and ¼ tsp. pepper. Roast 15 minutes. Vegetables will finish cooking in a later step. While vegetables roast, sear chicken.
Sear the Chicken
Heat a medium non-stick pan over medium-high heat. Add 1 tsp. olive oil and chicken to hot pan. Cook undisturbed until browned, 2-4 minutes. Transfer chicken, seared side up, to other half of baking sheet. Reserve pan; no need to wipe clean.
Finish Chicken and Make Sauce
Roast until vegetables are browned and chicken reaches a minimum internal temperature of 165 degrees, 4-6 minutes. While chicken roasts, return pan used to sear chicken to medium-high heat and add white wine and demi-glace. Don't be afraid of a little steam - reducing requires heat! Cook until reduced by half, 5 minutes. Remove from burner, and whisk in butter.
Finish the Dish
Place a bed of vegetables on plate and garnish with cranberries. Lay chicken breast next to vegetables, and sauce generously with white wine reduction. Sprinkle remaining parsley on chicken.