In a medium skillet or saucepan over medium heat, melt the butter.
Add the cubed butternut squash and diced onion. Cook, stirring occasionally, until the squash is fork-tender and the onion is soft and translucent, about 6 to 8 minutes.
Add the minced garlic and cook for 1 more minute, stirring constantly to prevent burning.
Add the Arborio rice and kosher salt. Stir to coat the rice with the butter and vegetables. Toast the rice for about 1 minute, until the edges of the grains look translucent and the center remains opaque.
Pour in the dry white wine. Reduce heat to medium-low and cook, stirring occasionally, until the wine has mostly evaporated, about 2 to 3 minutes.
Begin adding the hot chicken broth, about ½ cup at a time, stirring gently and continuously. Wait until the liquid is mostly absorbed before adding the next ½ cup of broth. Continue this process, adjusting the heat as needed to keep a gentle simmer, until all of the broth is incorporated and the risotto is creamy and tender with a slight bite in the center, about 30 to 35 minutes.
Add the heavy cream and finely grated Parmesan cheese. Stir until the cheese is fully melted and the mixture is smooth and creamy.
Remove from hea and garnish with fresh thyme and extra Parmesan cheese before serving.
Serve immediately for best texture, as risotto thickens as it sits.