Put the four tablespoons lemon juice into a small saucepan of salted water. Scrub the artichokes thoroughly but don’t peel them. Then slice the artichokes across very thinly and drop them into the salted water. Bring the water to the boil and allow it to bubble for 3–4 minutes, just until the artichokes are lightly cooked but still crunchy. Drain the artichokes and set them aside. Shortly before serving, slice the avocados in half, remove the stone, then peel off the skin. Slice the flesh thinly across in half circles, sprinkling them with lemon juice as you go. To assemble the dish, place the artichoke slices on a shallow serving plate or on to individual serving plates, and simply arrange the avocado slices on top. Sprinkle over the green peppercorns. Prepare the vinaigrette by putting all the ingredients into a sealed jar and shaking it well to combine. Just before serving, spoon the vinaigrette dressing evenly all over the artichoke and avocado slices and sprinkle the dish with finely chopped parsley.