1. For the dressing,combine mayonnaise,chives,parsley,tarragon,lemon juice,fish sauce (if using),salt and pepper in a blender and blitz until smooth.
2. In a mixing bowl,combine sour cream and herb mixture and mix gently to combine. Taste and adjust seasoning and store in refrigerator until required.
3. For the salad,combine lettuce,tomato,avocado,lemon juice,olive oil and salt in a bowl and gently toss to combine.
4. To serve,spoon the green goddess dressing into the centre of each serving plate and use the back of the spoon to make a circle. Arrange salad mix on top (scoop from the bottom of the bowl to get its olive-oil dressing);placing the large lettuce leaves on the bottom. Garnish with the sliced jalapeño and chives.