Trim asparagus and peel (see below). Cut into short pieces. Thinly slice one lemon half. Squeeze juice from other half.
Bring a large saucepan of salted water to a boil with the sugar and lemon slices. Add asparagus. Reduce heat to medium and simmer 15 minutes for white and 8 minutes for green asparagus, or until the spears are crisp-tender. Drain and leave to cool.
Trim excess fat from ham. Cut ham into wide strips. Place in a bowl with asparagus.
To make the dressing, whisk mayonnaise, yoghurt, oil and 1 tablespoon lemon juice in a bowl. Add the chervil and salt and pepper to taste. Stir dressing into asparagus and ham.
Peel eggs and cut into eighths. Arrange lettuce leaves on individual plates. Add asparagus and ham and garnish with egg slices.