01 - Preheat oven to 400°F. Line a rimmed baking sheet with parchment paper.
02 - Using a sharp knife, halve the spaghetti squash lengthwise. Scoop out and discard the seeds. Lightly brush the cut sides with olive oil if desired.
03 - Place squash halves cut-side down on the prepared baking sheet. Roast for 35–40 minutes until the flesh is fork-tender and strands pull apart easily.
04 - 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.
05 - Slowly whisk in the milk until smooth. Bring to a simmer, stirring often, and cook 2–3 minutes until the sauce thickens. Remove from heat and stir in cheddar, Parmesan, garlic powder, ground mustard, salt, and pepper until the cheese melts and the sauce is smooth.
06 - Use a fork to shred the roasted squash flesh into spaghetti-like strands. Transfer to a large bowl and gently toss with the cheese sauce until evenly coated.
07 - Transfer the coated squash to a greased baking dish. In a small bowl, combine panko, Parmesan, and melted butter. Sprinkle evenly over the squash. Broil on high for 2–3 minutes until the topping is golden brown.
08 - Serve warm, garnished with additional Parmesan or fresh herbs if desired.