Preheat the oven to 190C/170C Fan/Gas 5.
In a roasting tray, toss the squash, onion, pepper, tomatoes and garlic in the olive oil. Top with the thyme. Roast for about 45 minutes, or until all the vegetables are tender and lightly browned.
Discard the thyme and carefully transfer the roasted vegetables to a food processor. Blend to a thick purée. Pour 300ml/10fl oz of the stock into the food processor and blend again, until as smooth as possible.
Pass the puréed vegetables through a sieve into a large saucepan and stir in the remaining 200ml/7fl oz stock, the Parmesan and cream. Set aside while you prepare the garnish.
Heat a teaspoon of oil in a small non-stick frying pan and fry the pancetta or bacon for 3–4 minutes, or until crisp and nicely browned, turning once. Transfer to a plate lined with kitchen paper. Add the remaining oil to the pan and cook the sage leaves for 30–60 seconds, or until crisp. Put on the plate with the pancetta.
Reheat the soup gently, stirring constantly until hot. (Add a little water if the soup is too thick.) Season with salt and pepper to taste. Ladle into warmed bowls and drizzle with the remaining cream. Crumble the pancetta and sage leaves on top and serve.