01 - Preheat the oven to 400°F. Line a baking sheet with parchment paper.
02 - Cut the spaghetti squash in half lengthwise and scoop out the seeds. Place the halves cut side down on the prepared baking sheet. Roast for 35 to 40 minutes until the flesh is tender and easily shreddable with a fork.
03 - While the squash roasts, melt the butter in a saucepan over medium heat. Whisk in the flour and cook for 1 minute, stirring constantly to form a smooth roux.
04 - Gradually whisk in the milk until completely smooth with no lumps remaining. Add the Dijon mustard, garlic powder, onion powder, salt, and black pepper. Cook, stirring constantly, until the sauce thickens, about 3 to 4 minutes.
05 - Remove the saucepan from heat. Stir in the shredded cheddar and Gruyère cheeses until fully melted and the sauce is silky smooth.
06 - Once the squash is cool enough to handle, use a fork to shred the flesh into spaghetti-like strands. Discard the skins.
07 - Add the spaghetti squash strands to the cheese sauce and toss gently until evenly coated. Transfer the mixture to a greased 9x13-inch baking dish.
08 - In a small bowl, combine the gluten-free breadcrumbs with the melted butter and grated Parmesan. Sprinkle evenly over the squash mixture.
09 - Bake at 400°F for 10 minutes, or until the topping is golden and the sauce is bubbly around the edges.
10 - Remove from the oven, garnish with chopped fresh parsley, and serve immediately while hot.