01 - Preheat oven to 400°F. Line a rimmed baking sheet with parchment paper and set aside.
02 - Using a sharp knife, carefully halve the spaghetti squash lengthwise. Scoop out and discard the seeds and stringy pulp from the center.
03 - Place the squash halves cut-side down on the prepared baking sheet. Roast for 35 to 40 minutes until the flesh is fork-tender and the strands release easily when scraped.
04 - While the squash roasts, melt butter in a medium saucepan over medium heat. Whisk in the flour and cook for 1 minute to form a roux. Slowly pour in the milk while whisking constantly until smooth. Bring to a gentle simmer and continue stirring until the sauce thickens, about 2 to 3 minutes.
05 - Remove the saucepan from heat. Add the shredded cheddar, grated Parmesan, garlic powder, ground mustard, salt, and black pepper. Stir until all the cheese has melted and the sauce is completely smooth.
06 - Once the squash is cool enough to handle, use a fork to shred the flesh into spaghetti-like strands. Transfer the strands to a large bowl and gently fold in the cheese sauce until evenly coated.
07 - Transfer the coated squash mixture to a greased baking dish. In a small bowl, combine the panko breadcrumbs, Parmesan cheese, and melted butter. Sprinkle the mixture evenly over the top. Broil on high for 2 to 3 minutes until the topping is golden brown and crispy.
08 - Serve warm, garnished with additional grated Parmesan or fresh herbs if desired.