Lightly salt the cauliflower and leek pieces. Steam cook separately until tender, about 10 minutes each. When each is cooked, divide the vegetables between serving plates, set aside, and keep warm.
In the meantime, place the egg in a saucepan of cold water and bring to a boil. Continue to boil for 4 minutes. Drain. Peel the egg. Cut in half lengthwise. Set aside and keep warm.
To prepare the sauce, whisk the egg yolks and water together in a small saucepan over low heat until an emulsion is formed and it thickens enough to show the bottom of the pan while whisking. Whisk in the melted butter. Whisk in the lemon juice, curry powder, salt, and pepper.
Spoon the sauce over the vegetables. Sprinkle each plate with green onion pieces and ham. Set an egg half in the center. Decorate with a couple of parsley leaves.