6 medium leeks
2 tablespoon(s) olive oil
4 medium stalks celery, chopped
3 medium parsnips, peeled and chopped
2 medium red potatoes, cut into 1/2-inch pieces
1  (2-pound) butternut squash, cut into 1/2-inch pieces
2 can(s) (14- to 14 1/2-ounce) vegetable broth
4 cup(s) water
.5 teaspoon(s) chopped fresh thyme leaves plus thyme sprig for garnish
1 teaspoon(s) salt
.75 teaspoon(s) coarsely ground black pepper
1 cup(s) half-and-half or light cream