Position a rack in the upper third of the oven and preheat to 425 degrees . Line a baking sheet with parchment and grease the parchment. In a shallow bowl, toss the pork with the oil and soy sauce; let sit for 30 minutes.
On the prepared baking sheet, arrange the sweet potatoes and apples in 4 rows. Brush with the butter, then season with salt, pepper and the sugar. Bake until fork-tender, about 25 minutes.
Meanwhile, preheat a large skillet over medium-high heat. Add the pork and cook, turning once, until browned, about 5 minutes. Transfer to a plate and keep warm. Lower the heat to medium, add the flour to the skillet and cook, scraping up any browned bits, for 1 minute. Stir in 1/2 cup water until smooth and simmer for 2 minutes; season with salt and pepper. Serve the pork and gravy with the apples and sweet potatoes.