Preheat your oven to 400°F (200°C). Peel and cube the butternut squash and chop the apples. Toss them with olive oil in a large bowl, then spread on a baking sheet and bake for 25–30 minutes until tender and caramelized.

Thinly slice the fennel and onions. In a skillet over medium heat, add olive oil and sauté for 10–15 minutes until golden brown and soft.

Add the sweet Italian sausage to the skillet, breaking it into smaller pieces. Cook for about 8–10 minutes until browned and cooked through.

Layer the roasted butternut squash and apples in a large baking dish. Spoon the cooked sausage and caramelized fennel and onions over the top, then sprinkle grated Gruyere cheese on the surface.

Preheat your oven to 375°F (190°C). Bake the casserole for 20–25 minutes until the cheese is bubbly and golden brown.