For the dressing, mix the crushed garlic, buttermilk and mayonnaise until smooth. Stir in the chives and season to taste, adding a little lemon juice if you like. The dressing can be made up to two days ahead and chilled - just give it a quick whisk before using.
To serve, halve, stone, peel and slice the avocados and toss with lemon juice to prevent browning. Cut the iceberg or little gem lettuce into thin wedges. Scatter the lettuce, other salad leaves and avocado over a large serving dish. Drizzle with some dressing and serve the rest in a jug on the side.