To prepare the vichyssoise soup, melt butter in a heavy bottom pot over medium heat and add olive oil. When foaming subsides add leeks, onions and potatoes. Continue cooking, while constantly stirring, until the leeks and onions have cooked through. Add the chicken stock and bring to a simmer. Cook over medium heat until potatoes are completely cooked through. Remove pot from heat and allow the soup to cool to the touch.
Once the vichyssoise soup has cooled, carefully purée it in a blender or food processor. When the purée is smooth, pass it through a fine strainer into a clean pot. Add crème fraîche, heavy cream, milk, lemon peel, thyme, and bay leaf and return to a boil. Once the soup reaches a boil, remove the pot from heat and allow it to chill in an ice bath for a minimum of one hour. Strain the soup just prior to serving.
To make the scallop ceviche, combine scallops, salt, sugar, garlic, basil, chili, lemon verbena, lemongrass, coriander, lemon juice, and olive oil in a large non-reactive bowl and gently toss to season evenly. Chill the mixture, allowing the scallops to marinate for 20 minutes. Remove scallops from marinade and gently brush off excess ingredients. Toss marinated scallops with chopped cucumber.
To plate, fill small, chilled bowls with vichyssoise soup; top each with a spoonful of scallop ceviche and garnish with edible flowers.