Preheat oven to 350 degrees. Set spaghetti squash, cut side down, on a baking sheet. Roast for 30-40 minutes. Skin should pierce easily with a fork when done.
Remove from oven and allow to cool before flipping over. Using a fork, scrape the interior until spaghetti squash flesh easily comes out resembling long strands of pasta. Remove to a bowl and set side. Keep oven warm at 350 degrees. Set skins aside if you want to use them for serving.
Next, make a blond roux, combination of unsalted butter and flour. In a medium sauce pan, melt 1/2 cup unsalted butter. As it melts, the milk solids will rise to the top, a frothy white. Skim this off and then pour through a fine mesh sieve. Return the now clarified butter back to the sauce pan.
Whisk in flour over medium heat. Continue to whisk until the mixture is a loose paste, darkens slightly in color and starts to smell nutty. Remove before mixture turns tan or smells like popcorn (this would be a golden or dark roux).
Whisk in milk until smooth and then brie until smooth. Set aside and keep warm.
In a large skillet, melt butter over medium-high heat. Saute shallots and mushrooms until soft. Add to creamy brie sauce.
Add prosciutto to the same skillet and toss for 1-2 minutes, until just warm and slightly crispy.
Place raked spaghetti squash into your serving dish or empty skins. Top with a ladle of creamy brie sauce, generous sprinkle of crispy prosciutto, chopped parsley and Parmesan cheese.
If you've tried this recipe, come back and let us know how it was!
The amount of milk can be tricky based on 3 variables: water content in butter, gluten content in flour and fat content in milk. These will depend purely on the brand you buy and type (skim versus whole or even 2%). My advice would be to add milk until mixture is a thick cheese sauce consistency. I will not lie though, at first, it will be globby and you’ll think “this is never coming together!” but it will!