To prepare dressing: Combine 1 cup basil, ¾ cup yogurt, ¼ cup chives, 2 tablespoons lemon juice, 2 tablespoons oil, 1 tablespoon tarragon, 1 teaspoon anchovy paste, ¼ teaspoon plus ⅛ teaspoon salt and the chopped garlic in a mini food processor. Process until smooth and bright green, about 1 minute, scraping down the sides as needed.
To prepare salad: Toss 3 cups cabbage, 3 cups romaine, 1½ cups cucumber and 3 tablespoons scallions in a large bowl. Gently fold in diced avocado.
Drizzle the dressing over the salad; toss until evenly coated.
