01 - Preheat the oven to 400°F. Line a rimmed baking sheet with parchment paper and set aside.
02 - Using a sharp knife, carefully split the spaghetti squash in half lengthwise. Scoop out and discard the seeds and stringy membranes 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 pull apart easily with gentle pressure.
04 - While the squash roasts, melt the butter in a medium saucepan over medium heat. Whisk in the flour and cook for 1 minute to form a smooth roux. Slowly pour in the milk while whisking constantly to prevent lumps. Bring to a gentle simmer and cook, stirring continuously, until the mixture thickens, about 2 to 3 minutes.
05 - Remove the saucepan from heat. Add the cheddar, Parmesan, garlic powder, ground mustard, salt, and black pepper. Stir until all the cheese has melted and the sauce is silky and uniform.
06 - Once the squash is cool enough to handle, use a fork to scrape the flesh into long spaghetti-like strands. Transfer the strands to a large mixing bowl and pour the cheese sauce over top. Gently toss until every strand is evenly coated.
07 - Transfer the sauced squash to a greased baking dish. In a small bowl, combine the panko breadcrumbs, Parmesan, and melted butter. Sprinkle the mixture evenly over the surface. Broil on high for 2 to 3 minutes until the topping turns a deep golden brown.
08 - Let rest for 2 minutes, then serve warm. Garnish with additional Parmesan or fresh herbs if desired.