Preheat the oven to 350 degrees F.
Heat a medium saucepan over low to medium heat. Add in 2 tablespoons of butter minced garlic, stir and heat until fragrant. Once butter is melted, whisk in a little bit of the flour and then slowly add in milk, a little bit at a time, alternating with the flour and vigorously whisking away any lumps. Bring mixture to a boil, then reduce heat to a simmer and stir every so often, until the sauce thickens up.
Once the sauce has thickened up turn off the heat and stir in cheddar cheese and gruyere cheese, salt and freshly ground black pepper. Stir in the pumpkin puree.
Stir in the noodles and spinach until noodles are well covered and spinach has slightly wilted.
Add mixture to a 9X9 baking dish.
In a small bowl, mix chopped walnuts and breadcrumbs with melted butter. Sprinkle mixture on top of of the pasta and bake for about 25 minutes until breadcrumbs are golden brown.