Cut the squashin half, lengthwise and seed. Then cut crosswise into 1-inch slices and season with sea salt. Roast the slices, skin side down, in a 9 x 13 pan in oven until tender and golden, about 50 minutes.
Set aside 6 squash slices for serving with the risotto. Cut the flesh from remaining slices into 1/2-inch pieces, removing skin.
Start risotto after squash has been roasting 40 minutes:
Bring broth to a simmer and keep at a bare simmer, covered.
Saute onion in butter in a dutch oven (I use a cast iron enamel pot) over moderate heat, stirring, until softened, about 6 minutes. Add rice, garlic, and cumin and toast, stirring, 4 minutes. Careful with the cumin! It's a taste not everyone likes, and it can really overpower the dish. I almost omitted it, and I think I will next time.
Stir in 1/2 cup simmering broth and cook at a strong simmer (Med heat), always stirring! Stir until broth is completely absorbed! Can't stress this enough. You never want your risotto to get too wet during cooking.
Continue simmering and adding broth 1/2 cup at a time, until rice is creamy-looking but still al dente (it should be the consistency of thick soup), about 18 minutes total. (There should be broth left.)
Stir in squash pieces, cheese, salt, and simmer, stirring, 1 minute. (If necessary, thin risotto with some leftover broth.)
Serve risotto immediately, spooned over reserved squash slices.