First make the dressing. Whiz the herbs, garlic and scallions in a food processor. Then add the oil in a stream until you have a thick green sauce. Pour into a bowl and stir in the mustard and lemon juice. Season with salt and pepper.
Spread slices of freshly cut bread with a little butter and add a small pile of salad leaves to the bottom half. Drizzle over a little of the dressing and arrange plenty of ham on top. Add another good drizzle of the dressing then cover the sandwich with the top half of the bread.