Preheat your oven to 375°F / 190°C.
Prepare the squash: Carefully pierce the spaghetti squash in several places with a knife. Microwave on high for 3 minutes to soften it.
Slice off the stem and blossom ends. Stand the squash upright on a cutting board and cut it in half lengthwise. Scoop out the seeds.
Place the squash halves cut-side down on a rimmed baking sheet and pour 1½ cups of water onto the sheet.
Bake for 30 minutes, or until the squash is tender when pressed.
Create the squash noodles: Let the squash cool for 10 minutes cut-side up. Use a fork to scrape the flesh, which will naturally separate into spaghetti-like strands.
Transfer these strands to a large bowl.
Cook the pasta: While the squash cools, cook the spaghetti according to package directions.
Drain well and add it to the bowl with the spaghetti squash strands. Use two forks to gently toss them together.
Toast the walnuts: In a small, dry skillet over medium-high heat, toast the chopped walnuts for about 5 minutes, stirring frequently, until they are browned and fragrant.
Remove them from the skillet and set aside.
Make the rosemary breadcrumbs: Wipe out the skillet and heat the olive oil over medium heat.
Add the breadcrumbs and cook for 1 minute, stirring.
Add the minced garlic and chopped rosemary, and continue to sauté for 2 minutes until the garlic is fragrant and the breadcrumbs are golden brown.
Combine and serve: Add the toasted breadcrumb mixture to the bowl with the squash and pasta, and toss everything to combine.
Serve immediately, sprinkled with the toasted walnuts and grated Parmesan cheese.
