Preheat oven to 350 degrees F (175 degrees C). Lightly grease a 9x13-inch baking dish.

Slice ends off the squash, then cut widthwise into halves or rings. Use a spoon to scrape out the seeds. Place squash on rimmed baking sheet. Coat each piece with a little olive oil, salt and pepper. Bake for 30 minutes. Allow to cool.

Meanwhile, heat a large skillet over medium heat; cook and stir beef, sausage, fennel and zucchini until meat is browned, about 7 minutes.

Stir in 1/2 of the sauce and seasonings, then cover and set aside.

Peel the squash's skin away and separate the strands into long “spaghetti noodles”.

For the white mix: Whisk eggs, Parmesan cheese, 2 cups mozzarella cheese, ricotta cheese and butter in a large bowl. Set aside.

Mix squash 'noodles,' spinach, mayonnaise, sour cream, and remaining sauce and toss to coat.

Place half the noodle mixture into baking dish. Top with half the 'white mix' followed by the meat sauce. Repeat layers.

Pour reserved sauce over top, and sprinkle mixed cheddar and remaining mozzarella cheeses.

Cover and bake in preheated oven for 40 minutes.

Remove cover and continue to bake until the cheese is melted and lightly browned, 20 to 25 minutes longer.