Prep: Preheat the oven to 350℉. Line a sheet pan with parchment paper.
Roast spaghetti squash: Cut 1 spaghetti squash in half, lengthwise. Scoop out the seeds and discard. Place spaghetti, squash cut-side down, on the parchment and roast in oven on middle rack for 1 hour. Optional: rub 1 tablespoon butter and 1 tablespoon olive oil on the inside first and sprinkle with a little kosher salt. Let cool.
Cook meat mixture: About 10 minutes prior to the spaghetti squash being done, bring a large cast iron skillet to medium-high heat and add 1 tablespoon olive oil (or ghee). Add 1 tablespoon garlic and cook for approximately 1 minute, or until fragrant. Add 1 ½ pounds ground Italian sausage, red onion, and a pinch of kosher salt and continue to move around pan until fully cooked.
Option to add vegetables: if you choose to add a vegetable, add it now, and cook for a few more minutes until tender but firm.
Add sauce: pour 15 ounces marinara sauce of choice over the meat mixture, mix to combine, and continue cooking until heated through.
Assemble: Use a fork to shred some spaghetti squash into a bowl (top it with a chunk of ghee and a pinch of salt and pepper) and pile the meat sauce on top.
Serve: Enjoy immediately!
Notes
Optional Vegetables: you can easily add some vegetables to this meat sauce in step 3, like mushrooms, zucchini, fresh spinach, etc.
Meal Prep: this could easily be meal prepped! Just make the sauce and the spaghetti squash and store in separate containers, reheat, and enjoy throughout the week!
Storage: place in an airtight container for up to 5-6 days. Recommended to store the meat sauce separate from the cooked spaghetti squash to prevent any sogginess.