You will need to set your oven to 190°C or gas mark 5.
Butternut squash should be cut in half and the seeds should be scooped out. Slice the halves in half lengthwise and place them, cut side down, on the buttered parchment paper. In a preheated oven, roast the squash for about one hour, or until it is tender. Cool the squash until you can handle them after removing them from the oven.
A large pan should be heated with butter and oil. The onion should be cooked over medium heat, stirring occasionally, for 5-8 minutes, or until softened but not colored. Butternut squash flesh should be scooped out of the skin and added to the pan. The skin should be discarded. Add the thyme and stock cubes after cooking for 1-2 minutes. Stir until stock cubes have dissolved in 1 liter (1¾ pints) of boiling water. The soup should be brought to a boil and simmered for 20 minutes until the vegetables are tender. The soup should be allowed to cool slightly after being removed from the heat.
Season the soup with salt and freshly ground black pepper to taste and blend it until smooth.
Make sure the soup is reheated before serving. The mixture will be quite thick, so if you wish to thin it out, add boiling water to the desired thickness.
The soup can be served with croutons and thyme sprigs, topped with black pepper and croutons.