Build the pumpkin base. Add the garlic to the onions and cook for about 30 seconds, until fragrant. Add the pumpkin purée, Worcestershire, Dijon, thyme, nutmeg and freshly ground black pepper. Cook over medium heat for 3–5 minutes, stirring frequently, until the pumpkin thickens and deepens slightly in color.
Toast the rice. Add the Arborio rice and cook for 1–2 minutes, stirring frequently, until thoroughly coated and lightly toasted. Pour in the wine, scraping up any browned bits from the bottom of the pot. Cook until the wine is almost completely absorbed.
Cook the risotto. Add the warm beef stock about 3/4–1 cup at a time, stirring frequently and allowing most of the liquid to absorb before adding more. Continue cooking for 18–22 minutes, until the rice is creamy and al dente. If the rice is still too firm after adding the 4 cups of stock, add additional warm stock a little at a time as needed.
Make it cheesy. Remove from the heat. Stir in the Parmesan, 1 cup Gruyère and 3 tablespoons butter until melted, creamy and glossy. Taste and season with kosher salt and additional black pepper as needed. Add a splash of warm stock if necessary so the risotto is loose and flowing rather than thick or stiff.
Broil & serve. Transfer the risotto to a broiler-safe serving dish. Top with the reserved caramelized onions and remaining 1/2 cup Gruyère. Broil for 1–3 minutes, watching closely, until the cheese is melted, bubbling and golden in spots. Finish with fresh thyme and black pepper and serve immediately.