In a large, heavy pot or Dutch oven, melt the butter with the olive oil over medium-high heat. Add the onions and leeks. Cook, stirring, until the onions are soft and lightly golden, 8 to 10 minutes. Add the pumpkin, salt and bay leaves and saute until starting to soften. Add the chicken broth and herbs. Stir to mix. Bring to a boil, and then reduce the heat to medium and cook, uncovered, for 20 to 25 minutes, or until pumpkin is tender and broth is just beginning to thicken slightly. Once the pumpkin is tender, discard the herbs and puree the soup using an immersion blender.
In a medium saute pan, brown the sausage. Using a slotted spoon remove the sausage from the pan and drain on paper towel. Top the soup with the sausage. Garnish with the cheese and serve with warm crusty bread.