Gradually add noodles and salt to rapidly boiling water so that water continues to boil; cook, uncovered, stirring occasionally, until tender. Drain noodles in colander. Preheat oven to 400. Combine yogurt and cottage cheese; combine noodles, spinach and onion with cottage cheese mixture. Pour into a 1-quart baking dish; top with Cheddar cheese. Cover and bake 20-25 minutes. Uncover and bake until cheese is melted and brown.