Preheat oven to 425 degrees F (220 degrees C).

Arrange frozen Brussels sprouts in a single layer on a rimmed baking sheet; sprinkle apple and onion pieces evenly around the baking sheet. Drizzle the sprouts, apples, and onions with olive oil; sprinkle with salt, black pepper, and garlic powder. Toss the mixture gently to coat.

Roast in the preheated oven until sprouts are hot and fragrant, about 20 minutes. Sprinkle with lemon zest, and squeeze juice from zested lemon over the sprouts to serve.