Preheat oven to 350°F. Cut each squash in half lengthwise. Scoop out seeds, discard.
Place squash halves, cut sides up, on a baking sheet. Bake at 350 for 50
minutes. Let stand for 10 minutes.
Scrape inside of squash with a fork to remove spaghetti-like strands. Place strands in a clean dish towel; squeeze until barely moist.
Heat a large skillet over medium-high heat. Add oil to pan; swirl to coat. Add garlic, cook 30 seconds. Add spinach, cook 1 minute or until spinach wilts.
Remove from heat. Combine spinach mixture, squash strands, ricotta cheese, salt, and half of the mozzarella cheese in a medium bowl.
Return skillet to medium-high heat. Add turkey to pan, cook 4 minutes or until browned, stirring to crumble. Add marinara sauce, cover, reduce heat to medium, and simmer 4 minutes. Remove from heat.
Increase oven temperature to 425. Spoon sauce evenly into the bottom of each squash half. Top evenly with squash mixture. Sprinkle evenly with remaining mozzarella and parmesan. Bake at 425 for 20 minutes.
Preheat broiler to high (keep squash in oven). Broil squash 1 to 2 minutes or until cheese is golden brown and bubbly. Remove from oven, let stand 10 minutes.