1. For dressing, combine first four ingredients; mix well, then set aside.
2. Cook pasta according to package directions; drain and rinse with cold water; drain again.
3. Place in large bowl; add 1 to 2 Tbsp of the dressing; toss to coat.
4. Refrigerate pasta and remaining dressing until chilled.
5. To serve, add spinach, oranges, and onions to chilled pasta. Toss with remaining dressing.