Cut the squash in half and scoop out the seeds. Place cut side down on a baking sheet that has been lightly greased with olive oil and roast for 35 minutes. After squash is baked, reduce oven heat to 350 degrees.
Once cool enough to handle, remove strands of spaghetti squash and place in large bowl.
Prepare Casserole
While the squash is roasting, brown the sausage and onions together in 1 tablespoon olive oil, until the sausage is cooked through. Add in the garlic and oregano and saute for 1-2 minutes longer. Remove from heat and drain off excess grease.
Add sausage mixture, whisked eggs, and 2 cups of the sauce to the spaghetti squash and mix well.
Transfer mixture to an 8x8 dish and cover with remaining 1 cup of sauce. If using cheese, sprinkle evenly over the casserole at this time.
Bake for 1 hour at 350 degrees or until top is bubbly and browned.
Notes
Sausage: Use ground Italian sausage. Pork, chicken, or turkey all work well in this casserole. Just be sure to read the ingredients in your sausage as some brands have added gluten or sugar.
Spaghetti sauce--Be sure to omit sugar from homemade sauce or use no added sugar sauce to keep this Paleo Friendly.
Spaghetti squash: In place of roasting the squash, use my method for cooking spaghetti squash in the Instant Pot, by cutting in half, removing the seed, and cooking on high pressure for 7 minutes. Be sure to place your squash on a trivet over 1.5 cups of cold water.
Cheese: If you are not dairy-free or following a paleo diet, I suggest adding 1 cup of shredded Mozzarella and ¼ cup Parmesan cheese to the top of this casserole.