Heat a big pot over medium heat and melt 2 tablespoons of butter.
Add chopped onion and cook until they're soft, about 3-4 minutes.
Add minced garlic and Arborio rice. Cook for another 2-3 minutes, until the rice looks a bit see-through.
If using, pour in the white wine and let it cook until mostly absorbed.
Start adding warm broth, one ladleful at a time, stirring often. Allow the liquid to mostly absorb before adding more. This takes about 10 minutes.
Toss in the diced pumpkin and continue adding broth and stirring. This takes around 10 more minutes.
Once the rice is creamy and the pumpkin is tender, stir in grated Parmesan and diced Fontina cheese until melted and gooey.
Sprinkle in fresh thyme leaves and season with salt and pepper to taste.
Turn off the heat and stir in the remaining tablespoon of butter.