Stir together the first six ingredients for the risotto into a small pot and bring to a simmer.
Simmer for about 5 minutes. Remove mixture from the heat and pour into a blender with the broth, season with salt and pepper and blend.
With the motor running, add ¼ cup of butter into the blender, 1 tablespoon at a time. Blend until smooth, about 3 minutes.
Pour the mixture into a bowl and set aside until ready to use.
Pour the chicken broth into a medium pot and warm over low heat.
Place a large pan over medium heat and melt the remaining butter and olive oil.
Add the onion and garlic and sauté for about 3 minutes.
Add the rice and sauté for 3 to 5 minute or until each grain of rice is coated in oil and there is a white dot in the center of each grain.
Begin adding the warm broth to the rice, one ladle full at a time, stirring frequently.
Each time the liquid evaporates, add another ladle of broth and continue to stir.
Continue to add liquid and stir until the rice is al dente, 20-25 minutes.
Stir in the pumpkin puree until completely combined.
Stir in the mascarpone and parmesan until smooth. Fold in the thyme and season with salt and pepper.