Rinse pumpkins. Pierce tops deeply with a knife or sharp fork several times. Set pumpkins upright on a rack (they can be stacked) at least 1 inch above 1 1/2 inches water in a 14- to 15-inch wok or 6- to 8-quart pan. Cover wok and bring water to a boil over high heat. Keeping water at a boil, steam pumpkins until tender when pierced, 20 to 35 minutes, adding more boiling water to pan as needed. Remove pumpkins from pan.
When pumpkins are cool enough to handle, in about 5 minutes, with a small, sharp knife, cut off tops to make lids that will sit in place. Set lids aside. With a small spoon, scoop out and discard seeds without breaking pumpkin skins. To keep warm up to 40 minutes, set lids in place, return pumpkins to rack over water in pan, cover, and adjust heat so water barely steams.
Meanwhile, in a 3- to 4-quart pan over medium-high heat, frequently stir butter and onion until onion is limp, 4 to 5 minutes. Add rice and stir often until it begins to turn opaque, about 3 minutes.
Add 4 1/2 cups broth. Bring to a boil over high heat, stirring often. Reduce heat and simmer rice, stirring often, until tender to bite, 15 to 20 minutes. Add a little more broth if a creamier texture is desired. Stir in cheese. Add salt and pepper to taste.
At once, spoon risotto into hot pumpkin shells. Top with lids.