In a large bowl, combine the first six ingredients. In a small bowl, whisk buttermilk, mayonnaise, vinegar, sugar and salt. Pour over salad; toss to coat. Top with walnuts and green onions. Refrigerate leftovers.
To toast nuts, bake in a shallow pan in a 350° oven for 5-10 minutes or cook in a skillet over low heat until lightly browned, stirring occasionally.