Preheat your oven to 400°F (200°C). Toss the sweet potato cubes with olive oil, salt, pepper, and half of the chopped sage on a baking sheet. Spread them out in an even layer and roast for about 25-30 minutes until they are tender and slightly caramelized.
While the sweet potatoes are roasting, prepare the apples by coring and slicing them thinly. Set aside.
Once the sweet potatoes are ready and slightly cooled, transfer them to a mixing bowl. Gently mash with a fork or potato masher until mostly smooth but still with some texture.
Stir in the remaining chopped sage, cream, and grated cheese into the mashed sweet potatoes. Mix until well combined and season with more salt and pepper if needed.
Layer the sliced apples evenly over the bottom of a buttered baking dish. Pour the sweet potato mixture over the apples, spreading it out smoothly with a spatula.
Sprinkle a little extra cheese and sage on top for a flavorful crust. Bake uncovered in the preheated oven for 20-25 minutes until bubbling and golden on top.
Remove from the oven and let it rest for a few minutes. The casserole should be hot, set, and with a slightly crispy top layer.
Serve warm, scooping out hearty portions that showcase the layered sweet potatoes and apples with a fragrant sage topping.
