Preheat oven to 400 degrees. Line a rimmed baking sheet with parchment paper for easy cleanup.
To make the spaghetti squash easier to cut, using a sharp knife, poke several slits in a line around the squash where you plan to cut it.
Microwave the squash for 5 to 6 minutes (microwave each squash individually). Cool slightly before handling (or use a kitchen towel to remove from the microwave). The slightly softened squash will be much easier to cut in half.
On a cutting board, slice off the stem on each end and then slice the squash in half along the line of slits you made. Remove the seeds. Rub each squash half with olive oil and season to taste with salt and pepper.
Place squash halves cut-side down on prepared baking sheet. Roast until the inside of each squash half is tender and easily pierced with a fork, about 40 to 60 minutes (exact baking time will depend on your specific squash). Cool slightly and use a fork to fluff and remove the strands from the squash halves. Keep warm until serving time (the sauce will probably be done before the spaghetti squash is).
To make the meat sauce:
In a large saucepan or Dutch oven over medium-high heat, add beef, sausage, and onion and cook until mostly browned, about 5 minutes. Drain if desired. Stir in garlic until fragrant, about 30 seconds.
Stir in crushed tomatoes, tomato sauce, tomato paste, sugar, basil, Italian seasoning, fennel seeds, and bay leaf. Simmer uncovered for 30 minutes, stirring occasionally. Remove bay leaf and season to taste with salt and pepper (I like 1 teaspoon salt and 1/2 teaspoon pepper).
Serve warm spaghetti squash strands with the hot meat sauce. Garnish with parmesan cheese and fresh basil leaves if desired.
Spaghetti squash: Look for hard-skinned squash without any soft spots or damage. The squash should feel heavy (a sign of freshness). Store whole uncooked squash in a cool, dry place; it will usually last up to 2 months.
Ground beef: Substitute all or part of the ground beef with ground Italian sausage or ground turkey.
Sauce: Substitute a 24-ounce jar of store-bought sauce if you don't want to make your own (add it to the skillet with the browned beef and onion).
Sugar: Add sugar to taste. For some, that means omitting it entirely. For others, you can increase the sugar up to 1/4 cup.
Fennel seeds: These are important because they provide the bulk of the Italian flavor.
Yield: You should get 1 1/4 cups of "spaghetti" from each pound of spaghetti squash you buy.
Cutting spaghetti squash: Spaghetti squash can to be dangerously hard to cut, so it's best to soften it in the microwave first. See my full tutorial on How to Cook Spaghetti Squash for all the details.
Baking times may vary: Squash comes in all sizes, therefore cooking time may vary. Most average-sized squash should be ready in 40 to 60 minutes.
Make ahead: The meat sauce is a fabulous candidate for making ahead, so go ahead and triple the batch for meal prep or for freezing.
Freezing: Both the sauce and the cooked spaghetti squash freeze well. Let the cooked, cooled strands drain in a colander set over a mixing bowl overnight in the refrigerator. (Discard any water left behind.) Then package up, label, and date, pressing out any excess air. To eat, take it out of the freezer and thaw for 1 hour before reheating. Freeze the sauce separately, in case you're in the mood for real pasta someday.