Blend 1/2 cup mayonnaise, 1 chopped spring onion, 2 tablespoons lemon juice, 1/2 teaspoon kosher salt and a few grinds of pepper in a small food processor or blender until smooth. Spread some of the sauce on the bottoms of 8 toasted rolls. Top with thick-sliced baked or smoked ham, assorted pickled vegetables (such as carrots, cucumbers and peppers), watercress, more sauce and the roll tops.