Preheat oven to 375 degrees. Prepare the small pumpkins as though you were making jackolanterns, cutting off and reserving top, scooping out seeds and reserving, but stopping short of carving the face. Set aside. Repeat preparation with the medium pumpkin, then cut it into wedges along its natural grooves. Peel each wedge with a sharp swivel peeler. Cut flesh into 2-inch chunks and set aside.
In a small bowl combine ground spices with 1/2 teaspoon salt and 1/4 teaspoon pepper. Sprinkle this mixture into pumpkin cavities, dividing evenly. Cut 1 tablespoon butter into bits and divide between pumpkins. Replace pumpkin tops and place pumpkins in a shallow roasting pan. Roast 30 to 40 minutes until tender when pierced with a knife but still intact. Remove from oven and set aside.
While pumpkins are roasting, prepare the soup: Heat remaining butter along with olive oil over mediumhigh heat in large pot or Dutch oven until foamy. Add onions and remaining salt and pepper and cook 5 minutes until softened and begin to color. Add garlic and cook 1 minute to release flavor. Add diced potato, chunked pumpkin flesh and stock and bring to a simmer. Reduce heat and simmer 30 minutes until potatoes and pumpkin are completely soft. Pass soup through a food mill fitted with the fine blade into a clean pot. Whisk in cream, taste and adjust seasoning and set aside.
Roasted pumpkins and soup should be done at about the same time. To serve, remove tops from pumpkins and use a melon baller to scoop balls of pumpkin flesh, leaving them in their respective pumpkins. Carefully ladle soup into each pumpkin, filling about 2/3 full. Replace pumpkin tops, place each pumpkin on a plate, and serve.