Cut squash lengthwise in half; discard seeds. Place halves on a microwave-safe plate, cut side down. Microwave, uncovered, on high until tender, 15-20 minutes. Cool slightly., Preheat oven to 350°. Mix 1/4 teaspoon salt with remaining seasonings; add to beef, mixing lightly but thoroughly. Shape into 1-1/2-in. balls. In a large skillet, brown meatballs over medium heat; remove from pan., In same pan, heat oil over medium heat; saute onion until tender, 3-4 minutes. Add garlic; cook and stir 1 minute. Stir in collard greens, spinach, ricotta cheese and remaining salt; remove from heat., Using a fork, separate strands of spaghetti squash; stir into greens mixture. Transfer to a greased 13x9-in. baking dish. Top with tomatoes, meatballs, sauce and cheese. Bake, uncovered, until meatballs are cooked through, 30-35 minutes.