Step 1

Arrange a rack in the middle of the oven and heat to 400°F. Cut the squashes in half lengthwise and scoop out the seeds. Lay them cut-side down in a roasting pan or other baking dish, and add about an inch of water. Roast until completely tender soft when poked with a fork, 45 to 60 minutes. Transfer to a cooling rack until cool enough to handle.

Step 2

While the squash is roasting, heat the olive oil in a high-sided skillet or saucepan over medium-high heat until shimmering. Add the onions and cook until translucent, 5 to 8 minutes. Stir in the garlic and cook until fragrant, about 30 seconds. Add the beef and 1 teaspoon of salt. Cook until well-browned, breaking up the beef into small crumbles, 5 to 8 minutes. Stir in the crushed tomatoes and bring to a simmer. Continue simmering the sauce until the squashes are ready, 5 to 20 minutes. Taste and add more salt if needed.

Step 3

Use a fork to shred the inside of the each squash, leaving about a half-inch of squash left in the shell. Mix the shredded squash into the tomato sauce. In a separate bowl, mix together the ricotta cheese, parsley, and 1/2 teaspoon of salt.

Step 4

Wipe out the roasting pan and arrange the squash shells inside like bowls. Spoon 1/4 cup of the ricotta mixture into each shell and spread evenly over the bottoms. Spoon half of the tomato-squash mixture over the ricotta and spread into an even layer. Top with the remaining ricotta and tomato sauce, using up the rest of both.→ At this point, the shells can be covered and refrigerated for up to 24 hours. Add an extra 10 minutes to the cooking time.

Step 5

Bake for 15 minutes at 400°F. Sprinkle the mozzarella over the tops of the squashes and bake until the cheese is bubbly and browned in spots, 15 to 20 minutes more. (For a more golden top, run the squashes under the broiler for a minute or two.) Sprinkle parsley or basil over the tops and serve immediately.