Heat the oven to 400°F Heat the olive oil in a heavy-based, ovenproof dish. Fry the sausages for 3-4 minutes, turning frequently to allow the sausages to brown evenly. Remove from the pan and keep warm.
Add the butter to the pan with the pumpkin. Cook over a slightly higher heat until the pumpkin starts to colour. With a slotted spoon, remove the pumpkin from the pan and keep warm.
Add the onion and fry for 4-5 minutes until softened but not coloured. Add the garlic and cook for a further 1-2 minutes. Add the sage and the mustard and stir well.
Return the pumpkin to the pan with the stock and the butterbeans. Season generously with salt and freshly ground pepper and stir again. Bring to the boil, then reduce the heat to a gentle simmer.
Place the sausages on top of the pumpkin mixture. Cook in the oven for 15-20 minutes or until the sausages are cooked and the pumpkin casserole nicely thickened. Sprinkle with parsley and serve immediately.