Step 1

Preheat oven to 425°F. Cut the squash in half lengthwise and remove the seeds. Brush the flesh with 2 teaspoons oil. Place them cut side down on a baking sheet lined with parchment paper. Roast in the oven until tender, about 40 minutes. Remove from oven and cool. When the squash is cooled, scrape the flesh with a fork so that it forms spaghetti-like strands. You should have about 6 cups total.

Step 2

Meanwhile, make the sauce by heating 2 teaspoons oil in a large sauté pan over medium heat. Add the onion and garlic and cook for 4 to 5 minutes until partially softened. Stir in the tomatoes, Italian seasoning, red pepper flakes, ½ teaspoon salt, and ¼ teaspoon pepper. Simmer the sauce for 15 to 20 minutes until thickened.

Step 3

Meanwhile, mix the ricotta, egg, spinach, ½ cup of the mozzarella cheese, 1 teaspoon salt, and ¼ teaspoon pepper together in a large bowl. Squeeze all of the liquid from the spaghetti squash and add the squash to the ricotta mixture (the squash releases a lot of water as it cooks so make sure to squeeze it well). Stir to combine all of the ingredients.

Step 4

Turn the heat on the oven down to 375°F. To assemble the casserole, spread about 1½ cups of the sauce on the bottom of an 8 x 11-inch baking dish. Add the squash mixture on top and spread it out evenly. Spread the remaining tomato sauce over the top of the squash and sprinkle the remaining 1 cup mozzarella and Parmigiano-Reggiano cheese on top. Bake in the oven for 35 to 40 minutes until bubbly and cheese is melted. Garnish with parsley. Let stand for 10 minutes before cutting and serving.