Cook the cauliflower florets in a large saucepan of salted, boiling water until tender to the tip of a knife, about 3 minutes.
Drain well and add to a food processor. Pulse a few times until finely chopped.
In a large mixing bowl, whisk together the crème fraîche, mayonnaise, cream cheese, lemon juice and enough of the hot water to make a coating consistency. Season to taste with salt and pepper.
Pour about one-third of the mixture into a small bowl, stirring in enough of the remaining hot water to reach a pouring consistency. Cover and chill until ready to serve.
Add the cauliflower to the large mixing bowl and stir well to coat. Cover and chill until needed.
Blanch the nectarines and tomatoes in a large saucepan of boiling water for 20 seconds. Drain well and refresh immediately in iced water.
Drain again and pat dry. Cut the tomatoes in half horizontally and set aside.
Cut the nectarines in half and remove their pits. Dice four of the nectarine halves and add to the cauliflower, stirring well to incorporate.
Arrange metal ring molds on serving plates and fill with the cauliflower mixture, packing down with the back of a tablespoon.
Sit the nectarine halves on top, followed by the tomato halves and a black olive. Cover and chill for at least 1 hour.
To serve, pour the prepared sauce carefully around the terrines. Garnish with the black olives halves and oregano sprigs before serving.