Arrange the oven rack to the upper half. Preheat the oven to 400F (204 C). Line a baking sheet with parchment paper.
Bring a large pot of salted water to a boil over high heat.
While the water is heating up, scrub and rinse the potatoes and apples to remove any dirt. On a study flat surface, cut the potatoes and apples in half lengthwise, then lay the halves cut side down.
Cut the halves into thirds so you end up with wedges. For the apples, cut the inner core and seeds out.
Once the water is boiling, turn the heat off. Add the potato wedges to the hot water and let them soak for 10 minutes.
Drain the potatoes in a colander and let them air dry for a few minutes. Shake the colander a few times to rough up the edges of the potato wedges.
In a large bowl, add the potato wedges, apple wedges, salt, pepper, garlic powder, parmesan cheese and rosemary.
Toss the potatoes and apples together until evenly coated in the oil and seasonings.
Pour the potato mixture in a single layer onto the prepared baking sheet and evenly space out the potatoes and apples, ensuring each piece has a side touching the baking sheet.
Place the baking sheet into the preheated oven. Roast the potatoes and apples for 30-45 minutes, flipping halfway through, until golden brown and crispy.
If the potatoes don't look crispy enough to your liking, you can leave them in the oven for another 5-10 minutes to get extra crispy, just keep your eye on them so they don't burn!