Cut the spaghetti squashes in half length wise and scoop out the seeds.
Lay them cut-side down in a roasting pan or other baking dish, and add approximately one inch of filtered water. Roast for 45-60 minutes, or until soft when poked with a fork.
While the squash is roasting, warm the olive oil in a high-sided skillet or saucepan over medium-high heat. Stir in the onions and cook until translucent, approximately 5 minutes. Stir in the garlic and cook until fragrant, about 30 seconds.
Add the ground beef and 1 teaspoon of sea salt. Cook until the ground beef is well-browned, approximately 5-8 minutes.
Stir in the crushed tomatoes and bring to a simmer. Continue simmering the sauce until the squashes are ready, 5-20 minutes. Taste the sauce and add more salt if desired.
Use a fork to shred the inside of the spaghetti squash, leaving about a half-inch of squash in the shell. Mix the shredded squash into the tomato sauce. In a separate bowl, mix together the ricotta cheese, parsley and remaining ½ teaspoon of salt.
Empty and wipe out the roasting pan, then arrange the squash shells inside, like bowls.
Use a spoon to spread ½ cup of the ricotta evenly over bottom of each shell.
Spoon a ¼ of the tomato-squash mixture on the top of the ricotta cheese.
Top with another layer of ricotta and tomato sauce, using the remainder of each.
***If making this recipe in advance, cover the shells at this point with foil and refrigerate for up to 24 hours. Add an extra 10 minutes to the cooking time.
Bake shells for 15 minutes. Sprinkle the shredded mozzarella over the tops of the squashes and bake for another 15-20 minutes, until the cheese is melted and starting to brown. For a more golden top, broil the squashes for up to 2 minutes.
Garnish with parsley or basil and serve with a green salad. Leftovers will keep for up to 1 week in the refrigerator.
This recipe can be made into a casserole version (leaving out the shells).Casserole Version: In a 2-quart baking dish, layer 1/3 of the tomato-squash mixture into the dish and top with half of the ricotta. Continue building layers, ending with the last third of the tomato-squash mixture. Bake for 15 minutes, and then sprinkle with mozzarella. Bake for 15-20 minutes, until bubbly.