Preheat oven to 425°F. Line a large rimmed baking sheet with parchment. Grate 2 tsp zest from the lemon (reserving lemon for later).
Toss the Yukon and sweet potatoes with 2 tbsp oil and 1 tsp garlic powder. Arrange on baking sheet, spreading around edges. Season with salt.
Rub the chicken with lemon zest, remaining 1 tbsp oil, and remaining 1 tsp garlic powder. Place in center of prepared pan and season with salt and pepper. Place in oven and bake 30–35 min., until chicken is cooked and potatoes are tender.
To serve, squeeze juice from lemon on chicken and potatoes. Garnish with the parsley.