Preheat the oven to 200°C fan. Place the butternut squash, onion and garlic in a large, shallow casserole dish and spray well with low-calorie cooking spray. Toss to coat evenly.
Place in the oven and roast for 30 minutes, turning halfway through, or until the butternut squash is tender and the onion is soft.
Add the cream cheese to the hot stock and stir until completely dissolved. Stir in the milk, onion granules, Cheddar cheese, mustard and sage. Season with freshly ground black pepper to taste.
Put the gnocchi in the casserole dish, pour over the liquid mixture and stir gently. Place in the oven, uncovered, and bake for 20 minutes until bubbling and golden brown. Serve.