Spaghetti Squash Casserole

Preheat oven to 425°F. Rub cut sides of squash evenly with 3 tablespoons of the oil; sprinkle with 1 teaspoon of the salt and 1/2 teaspoon of the pepper. Arrange, cut sides down, on a baking sheet lined with parchment paper. Roast in preheated oven until easily pierced with a knife, 40 to 45 minutes. Remove from oven. Reduce oven temperature to 350°F. Let squash cool 10 minutes. 

While squash cools, heat remaining 1 tablespoon oil in a large skillet over medium-high. Add beef; cook, stirring often, until just browned, 4 to 5 minutes. Add onion and garlic; cook, stirring often, until softened, 3 to 4 minutes. Stir in marinara and remaining 1 1/2 teaspoons salt and 1/2 teaspoon pepper. Cook, stirring constantly, 1 minute. Remove from heat.

Using a fork, scrape cooled squash flesh into large strands, and place in a large bowl. Discard shells. (Strands may be stored in an airtight container in refrigerator up to 2 days.) Add beef mixture to squash in bowl; toss to combine. Stir in half (about 2 cups) of the mozzarella and half (about 1/4 cup) of the Parmesan. 

Spoon mixture into a 13- x 9-inch baking dish. Sprinkle evenly with remaining 2 cups  mozzarella and 1/4 cup Parmesan. Bake at 350°F until cheese is lightly browned, 30 to 35 minutes. Let cool 5 minutes. Sprinkle with parsley. Serve.