8 ounces uncooked small pasta shells
1 cup fat-free mayonnaise
1/4 cup grated Parmesan cheese
1/4 cup buttermilk
1/3 cup finely chopped onion
2 tablespoons minced fresh parsley
1-1/2 teaspoons minced fresh basil
1/2 teaspoon salt
1/2 teaspoon pepper
1 pound cooked medium shrimp, peeled and deveined
1/4 cup frozen peas, thawed
1/4 cup diced pimientos
1 medium tomato, seeded and chopped
2 green onions, thinly sliced