Preheat the oven to 425°F (218°C) and line a baking sheet with foil, lightly misted with cooking spray.
In a large bowl, combine the cubed butternut squash and apples. Drizzle with about 2 tablespoons of olive oil, sprinkle with salt and pepper, 1 tablespoon of maple syrup, ground sage, and Herbes de Provence. Toss to coat evenly.
Spread the mixture onto the prepared baking sheet and roast for 35 minutes, stirring occasionally, until the squash is tender and golden.
While the squash and apples are roasting, heat a drizzle of olive oil in a large skillet over medium-high heat. Add the crumbled sweet Italian sausage and cook until browned and cooked through. Remove from the skillet and set aside.
In the same skillet, add more olive oil if needed, then add the sliced fennel and onion. Cook, stirring frequently, for 10-12 minutes, or until caramelized and jammy in texture. Remove from heat and combine with the cooked sausage.
Once the squash and apples are tender, switch the oven setting to “broil.”
In a large bowl, combine the roasted squash and apples with the sausage, fennel, and onion mixture. Add the remaining 1 tablespoon of maple syrup and toss to combine. Taste and adjust seasoning with salt and pepper as needed.
Transfer the mixture into a medium-sized (9×13) baking dish. Top with grated Gruyere cheese.
Place the casserole under the broiler for a few minutes, until the cheese is melted and golden-brown.
For garnish, fry sage leaves by heating ¼ cup olive oil in a small saucepan. Once hot, add the sage leaves and fry for about 20 seconds. Remove and drain on paper towels, sprinkle lightly with salt.
Garnish the casserole with the fried sage leaves (or fresh ones) and serve warm.