In a medium-size heavy saucepan, heat the olive oil over medium-high heat. Add the onion and garlic and saute, stirring continuously, just until softened, 3 to 4 minutes. Add the rice and continue to stir, using a wooden spoon, to coat the rice with the oil.
Add the white wine and continue cooking, stirring often, until it has been absorbed by the rice. Pour in enough chicken stock to cover the rice completely, about 3 cups, and continue to cook, stirring often, until all the liquid is absorbed.
Pour in 1 cup more of the remaining stock and stir and cook until it has been absorbed. Repeat with 1 more cup. Add the remaining cup and cook, stirring, until the rice is al dente, tender but still very chewy, and most of the liquid has been absorbed.
Stir in the pumpkin puree and the diced pumpkin and reduce the heat to very low so that the risotto doesn't simmer anymore. Stir in the Parmesan and butter to give the risotto a nice, creamy finish. Spoon it immediately into heated shallow serving bowls.