Whisk together mayonnaise, sour cream, lemon juice, anchovy paste (if using), garlic, salt, and pepper. Stir in finely chopped parsley, chives, and tarragon until the dressing is pale green with herb flecks. If too thick, thin with a tablespoon of cold water.

Shred romaine, dice cucumber, thinly slice radishes and green onions, and thaw peas under cold water. Pat vegetables dry with a towel.

In a large bowl, toss the romaine, cucumber, radishes, peas, and green onions with half the dressing until evenly coated.

Transfer to a platter or bowls. Slice the avocado, arrange it on top, and drizzle the remaining dressing over everything. Finish with black pepper and a few fresh herb leaves, then serve immediately.