Cook the butternut in a large saucepan of lightly salted boiling water for 10 minutes, or until cooked through and it can be easily pierced with a knife.
Meanwhile, preheat the oven to 190 °C.
Heat the butter and olive oil in a large pan over medium heat and cook the onion for 7–8 minutes, or until translucent. Add the garlic and sage, and cook for another 2–3 minutes.
Lightly grease a small to medium-sized shallow ovenproof dish and to it add the butternut, onion mixture, cheese and chicken. The filling should fit snugly and be quite deep – this will give you a moister crumble. Season to taste.
In a separate bowl, mix the crumble topping ingredients together until it has the consistency of rough breadcrumbs. Scatter this over the filling and place the dish in the oven for 20–25 minutes, or until golden. Garnish with fresh sage.