Preheat oven to 350 degrees F.
Roast whole miniature pumpkins in a small roasting pan with 1/2-inch water, tightly covered with foil, until very tender, 45 to 50 minutes. Cool slightly. Cut out tops and reserve, then scoop out seeds. Discard water from roasting pan, then return pumpkin shells to pan and keep warm, covered with foil.
Meanwhile, make risotto while pumpkins roast: Cook diced pumpkin in a medium saucepan two-thirds full of simmering water until tender, 2 to 3 minutes. Drain in a colander.
Bring stock and 2 cups water to a simmer in a small saucepan and keep at a bare simmer. Cook onion in oil in a 2 to 2 1/2-quart heavy saucepan over moderate heat, stirring occasionally, until softened, about 3 minutes. Add rice and cook, stirring, 1 minute. Add 1 cup simmering stock and cook at a strong simmer, stirring constantly, until stock is absorbed. Continue simmering, adding stock 1/2 cup at a time, stirring constantly and letting each addition be absorbed before adding the next, until rice is tender and creamy-looking but still al dente, about 18 minutes total. (There may be broth left over.)
Remove from heat and stir in diced pumpkin, cheese, and butter, stirring until butter is melted. Season with salt and pepper, and cover to keep warm.
Put each pumpkin shell on a plate, fill with risotto, and cover with reserved tops.