01 - Preheat oven to 400°F. Halve the spaghetti squash lengthwise and scoop out the seeds. Place cut side down on a parchment-lined baking sheet. Roast for 35 to 40 minutes until the flesh is tender and shreds easily with a fork.
02 - While the squash roasts, melt the butter in a medium saucepan over medium heat. Whisk in the flour and cook for 1 minute, stirring constantly to form a smooth roux.
03 - Gradually pour in the whole milk while whisking continuously. Continue cooking for 2 to 3 minutes until the mixture thickens enough to coat the back of a spoon.
04 - Reduce the heat to low. Add the cheddar, Gruyère, garlic powder, ground mustard, salt, and pepper. Stir until all the cheese has melted and the sauce is completely smooth. Remove from heat.
05 - Once the roasted squash is cool enough to handle, use a fork to scrape the flesh into long strands. Transfer the strands to a large mixing bowl and gently fold in the cheese sauce until evenly coated.
06 - Transfer the sauced squash to a lightly greased baking dish. In a small bowl, combine the breadcrumbs, Parmesan, and melted butter. Sprinkle the mixture evenly over the top. Bake at 400°F for 10 to 12 minutes until the topping is golden and bubbling.
07 - Remove from the oven and garnish with fresh chopped chives or parsley. Serve immediately while hot.