Fill 3-quart saucepan 2/3 full of water; heat to boiling. Add Pasta. Gently boil, uncovered, 12 minutes, stirring occasionally; drain. Rinse with cold water to cool; drain well.
Meanwhile, in large bowl, combine Seasoning mix from packet, mayonnaise, sour cream, blue cheese and lemon juice. Stir in remaining ingredients except spinach. Refrigerate at least 1 hour.
Just before serving, place 2 cups of the spinach on each of 4 plates; divide pasta salad evenly over spinach.