Heat oven to 400°F (or 375°F convection)
Cut the squash: wash, then carefully cut crosswise across the middle. For longer strands, cut across the equator. For shorter strands, cut from pole to pole.
Scoop out the seeds and the stringy part in the middle. You likely won't get all the stringy part, but do your best to get all the seeds.
Season lightly: brush the cut sides with your melted butter or olive oil, add salt, pepper, and optionally add garlic and italian seasoning.
Roast: Place halves cut-side down on a lined sheet pan. Roast 30-40 minutes.
Check doneness: fork should go in easily, but squash should not collapse. Overcooking = mushy, watery strands.
Shred: cool 5-10 minutes, then scrape with a fork to release strands.
Dry the strands (important): put in a colander 5 minutes, then gently press with paper towels or spread on the sheet pan and bake 8-10 more minutes to evaporate moisture.
Optional: Toss dried spaghetti squash noodles with 1 Tbsp butter
Serve: Warm your spaghetti sauce separately and combine at the end. Do not simmer the squash in the sauce or it will turn watery and soft.
Finish: Grate fresh parmesan over the top of sauce and noodles (best) or add parmesan from a shaker bottle (good).