Preheat your oven to 180°C (350°F) and cook the pasta in salted boiling water until just al dente, about 8 minutes. Drain and set aside.
In a large skillet, melt butter over medium heat. Add the diced onion and cook until translucent and fragrant, about 5 minutes.
Stir in the pumpkin purée and a pinch of nutmeg. Let it simmer gently until warmed through and combined, about 3 minutes, filling the kitchen with a cozy aroma.
In a separate pan, make a roux by melting the remaining butter, then whisking in flour until smooth and slightly bubbly. Gradually pour in the cream, whisking constantly until the sauce thickens and becomes silky.
Add the shredded cheddar and Gruyère cheeses to the pumpkin mixture, stirring until melted and creamy. Pour this cheese sauce into the skillet with the onion and pumpkin, stirring well to combine.
Fold in the cooked pasta, ensuring each piece is coated with the cheesy pumpkin sauce.
Transfer everything into a baking dish and sprinkle the breadcrumbs evenly over the top.
Bake in the preheated oven for 20-25 minutes, until the top is golden brown and bubbling around the edges.
Remove from oven and let rest for about 5 minutes, allowing the sauce to set slightly and flavors to meld.
Scoop into bowls, serve hot, and enjoy the comforting blend of cheesy pumpkin goodness with a crispy topping.