Position a rack in the top third of the oven and heat the oven to 475°F.
In a large bowl, toss the sweet potatoes with the oil, 1 tsp. salt, and a few grinds of black pepper. Spread the potatoes in a single layer on a large, rimmed baking sheet. Roast for 10 minutes; then flip the potatoes and continue roasting until tender and browned in spots, about 5 minutes more.
While the potatoes roast, heat the butter in a 12-inch skillet over medium-high heat until melted and beginning to brown, 1 to 2 minutes. Add the apples in a single layer and brown on both sides, 1 to 2 minutes per side. Add the sage and stir until wilted, about 30 seconds. Off the heat, stir in the lemon juice and maple syrup. With a spatula, scrape the apples and butter into the bowl used for tossing the potatoes.
When the potatoes are done, add them to the apples and gently combine with the spatula. Season to taste with salt and pepper, and serve.