Heat oven to 425. Toss the cubed potato and apple in one tablespoon of olive oil on a parchment lined rimmed metal baking sheet. Roast for 30 minutes or until tender when pierced with a fork, flipping after 15 minutes.
Meanwhile, cook the onion in the remaining tablespoon of oil in a medium skillet over medium-low heat, until very soft and tender.
Gently toss the roasted potatoes and apples with the cooked onion, along with the thyme, black pepper and salt, to taste.