Split squash. Remove seeds and stringy portion. Use fork to comb squash flesh, pullin it off in long strands. Place in large mixing bowl. Toss with four, garlic, and salt until well combined.
Blend Ricotta, spinach, and Parmesan cheese in food processor or blender until smooth.
To assemble, spread 1/2 cup pasta sauce on bottom of a shallow 6 cup baking dish. Top with spaghetti squash, arranging evenly and compactly. Spread Ricotta mixture evenly over squash. Spread remaining sauce over Ricotta. Cover with sliced Mozzarella.
Bake on center rack of 350 degree oven, uncovered, until cheese is lightly browned on edges and juices bubble, about 40 to 50 minutes. Cut into squares and serve hot. Makes 6 servings.