Cook pasta according to package directions. Meanwhile, for dressing, combine the mayonnaise, Parmesan cheese, buttermilk, onion, parsley, basil, salt and pepper in a blender or food processor; cover and process until blended
Drain pasta and rinse with cold water. In a large bowl, combine the pasta, shrimp, peas, pimientos, tomato and green onions. Add dressing and toss to coat. Cover and refrigerate until chilled.