In a large pot, heat the butter over medium-high heat. Add the prosciutto, onion, garlic and thyme and fry for 3 minutes or until onion and prosciutto are slightly browned.
Add pearl barley, pumpkin and sweet potato and cook for 5 minutes. Add vegetable stock. Bring to the boil, then reduce the heat to a simmer and cook for 1 hour, covered, until pumpkin, sweet potato and barley is tender. Stir occasionally to prevent sticking to the bottom of the pot.
With a fork, mash the pumpkin and sweet potato to a desired consistency.
Fry the remaining sliced prosciutto until crunchy. Serve on top of soup, sprinkle with pine nuts and season with salt and pepper.