Step 1

Arrange a rack in the middle the oven and heat to 400°F. Line a rimmed baking sheet with aluminum foil; set aside.

Step 2

Cut each squash in half lengthwise and scoop out the seeds. Brush the cut sides of each squash with the oil and season with the salt and pepper. Place cut-side down in a single layer on the baking sheet, placing one garlic clove under each piece of squash. Roast until the squash are soft when poked with a fork and the skin is browned and blistered, about 40 minutes.

Step 3

Remove the baking sheet from the oven. Flip the squash over with tongs and remove and discard the garlic. Use a fork to shred the inside of each squash half, leaving about 1/2 inch of unshredded squash left in the shell. Transfer the shredded squash to a large bowl. Add the sauce, Parmesan, basil, and red pepper flakes and stir to combine.

Step 4

Evenly divide the shredded filling between the squash halves. Divide the meatballs over the filling, then top with the mozzarella cheese.

Step 5

Bake until the meatballs are heated through and the cheese is melted, 15 to 20 minutes. Garnish with more fresh basil before serving.