Prepare squash: Preheat oven to 400°F. Rub squash evenly with 2 tablespoons of the oil, 1 teaspoon of the salt, and ½ teaspoon of the pepper.

Flip squash: Arrange squash, cut sides down, on a large rimmed baking sheet lined with parchment paper.

Roast spaghetti squash: Roast in preheated oven until squash is easily pierced using a knife, about 45 minutes. Remove from oven; reduce oven temperature to 350°F. Let squash cool for 10 minutes.

Cook ground beef and sausage: While squash cooks, heat remaining 1 tablespoon oil in a Dutch oven over medium-high. Add ground round and sausage; cook, stirring often, until just browned, about 6 minutes.

Add onion and garlic: Add onion and garlic to Dutch oven; cook, stirring often, until softened, about 4 minutes.

Add marinara: Stir marinara and remaining ¼ teaspoon salt and ½ teaspoon pepper into mixture in Dutch oven. Cook, stirring constantly, about 1 minute. Remove from heat, and set aside.

Scrape squash: Using a fork, scrape cooled squash flesh into long strands, and place in a large bowl. Discard shells. (Strands may be refrigerated in an airtight container up to 3 days.)

Mix squash and meat mixture: Add beef mixture to squash in large bowl; toss to combine.

Add cheese: Stir half (about 1 cup) of the mozzarella cheese and half (about ¼ cup) of the Parmesan into mixture in large bowl.

Add casserole mixture to baking dish: Spoon mixture into a 13- x 9-inch baking dish. Sprinkle evenly with remaining 1 cup mozzarella and ¼ cup Parmesan.

Bake casserole: Bake at 350°F until cheese is melted and lightly browned, about 35 minutes. Let cool 5 minutes.

Top with parsley: Sprinkle casserole evenly with parsley. Serve.